CBSE Class 7 Science Advanced Physics Light Notes Set 01

Light

Seeing Things: Light, Luminous and Non-luminous Bodies

We see an object only when light coming from it reaches our eye. Light is a form of energy that gives us the sensation of sight. In daytime, sunlight falls on things around us, bounces off them and enters the eye; that is how we see a cricket ball or the words on this page.

Not everything we see produces its own light. A luminous (self-luminous) body gives out light of its own, for example the Sun and the other stars. A non-luminous body gives out none; it is seen only because it throws back (reflects or scatters) the light that falls on it, as the Moon and the planets do. Some living things, such as fireflies, glow-worms and certain fish, make their own light; this is called bioluminescence. A candle is non-luminous when unlit and becomes luminous once it is burning.

Anything that gives out light is a source of light. Natural sources include the Sun, the stars and the glow-worm; artificial (man-made) sources include electric bulbs and lamps.

Optical Media

Any material through which light can travel, wholly or partly, is an optical medium. A medium with the same composition everywhere is homogeneous; one whose composition changes from point to point is heterogeneous (the book gives air and impure water as examples of the latter).

On the basis of how much light they pass, media are of three kinds.

Fig. 4.1 Three kinds of media according to the amount of light they pass

Kinds of Sources

By size, a source is either a point source (about as small as the head of a pin) or an extended source, which is bigger, such as a tube light or a burning candle.

Fig. 4.2 Point source and extended source of light

By the way light is produced, a source is either a hot source, which glows only when it is very hot (the Sun, a candle flame, a filament bulb), or a cold source, which gives light even when cool (a glow-worm). The same object can be called hot or cold depending on how its light is produced.

Hurricane Lamp and Electric Bulb

A hurricane lamp has a small kerosene tank with a burner at the bottom. A wick dips into the kerosene, which climbs up the wick by capillary action to feed the flame. The glass chimney protects the flame from wind and spreads the light in all directions. Holes at the burner let cool air in, and holes at the top let hot gases out.

The electric bulb, invented by Thomas Alva Edison, is the commonest source of light. A current passing through its thin filament makes the filament white-hot so that it glows, changing electrical energy into light energy. The air is taken out of the bulb and a little inert gas such as helium is filled in so that the hot filament does not burn away.

Rays and Beams of Light

To study light we draw its path as a straight line with an arrow; this is a ray of light. A group of rays is a beam. Rays that stay side by side at the same distance form a parallel beam (rays from a very distant source are almost parallel). Rays that come together at a point form a convergent beam, and rays that seem to spread out from a point form a divergent beam.

Fig. 4.4 to 4.7 A ray, a parallel beam, a convergent beam and a divergent beam

Rectilinear Propagation of Light

In a homogeneous medium light travels in a straight line. This is called the rectilinear propagation of light (light takes the shortest path between two points). To check it, take three identical cardboards with a hole at the middle of each and stand them so that the holes lie on one straight line. A lighted candle can then be seen through all three holes. If any one cardboard is moved sideways, the candle disappears from view. Likewise we can look through a straight pipe but not through a bent one.

Fig. 4.8 Light travels along a straight line

Shadows and eclipses are both explained by this straight-line travel.

Shadow

A shadow is the dark patch that forms behind an opaque object placed in the path of light. With a point source P, an object OJ throws a sharp shadow SH on the screen AB.

Fig. 4.9 Formation of a shadow by a point source

When the source is extended, the shadow has two parts: a fully dark central part, the umbra (SH), and a partly dark border on each side, the penumbra (HH₁ and SS₁). Umbra and penumbra appear only for an extended source; a point source gives an umbra alone. Their sizes depend on the size of the source, the size of the object, the distance from source to object and the distance from object to screen.

Fig. 4.10 Umbra and penumbra formed by an extended source

Eclipse

The Sun, the Earth and the Moon sometimes line up, and then one of them falls in the shadow of another. If the Earth's shadow falls on the Moon we have a lunar eclipse; if the Moon's shadow falls on the Earth we have a solar eclipse.

Lunar Eclipse

On a full-moon night the Earth lies between the Sun and the Moon. If the three are in a straight line, the Earth's shadow covers part or all of the Moon for a while as seen from some places. If only part of the Moon is hidden it is a partial lunar eclipse; if the Moon is hidden completely it is a total lunar eclipse. A lunar eclipse can occur only on a full-moon night.

Solar Eclipse

On a new-moon day the Moon lies between the Sun and the Earth. If the three are in a line, the Moon's shadow falls on the Earth and a solar eclipse occurs there; so a solar eclipse occurs only on a new-moon day. If the Sun is hidden completely it is a total solar eclipse; if part of the Sun is still visible it is a partial solar eclipse. When only the tip of the Moon's umbra reaches the Earth, the Moon appears slightly smaller than the Sun and a bright ring of the Sun is seen around it; this is an annular solar eclipse. For a moment the Sun's corona can show up as a bright ring, called the 'diamond ring'.

An eclipse does not happen every new-moon or full-moon day, because the Moon's orbit round the Earth is tilted with respect to the Earth's orbit round the Sun, so the three bodies are not always in one line. A solar eclipse must never be looked at with bare eyes; use proper protective filters.

Pinhole Camera

A pinhole camera forms a photograph-like picture of a stationary bright object. It is a closed box (cubical or cylindrical) with a tiny hole in one face and a paper or glass screen on the opposite face, blackened inside so that no stray light is reflected. Light from each point of the object goes in a straight line through the hole and lands on the screen, so an inverted image A₁B₁ of the object AB is seen. For an actual photograph the screen is replaced by a photographic film and the box is kept covered with black cloth, which is removed only on the side of the pinhole for the exposure.

Fig. 4.11 A pinhole camera (u = object distance, v = image distance)

The size of the image depends on the distance of the object from the pinhole and the distance of the screen behind it. The comparison of the sizes is given by the magnification:

\[ m = \frac{i}{o} = \frac{v}{u} = \frac{\text{width of the camera}}{\text{object distance}} \]

Here i is the size of the image, o the size of the object, v the image distance and u the object distance.

Example. A candle 6 cm tall is 2 cm from a pinhole camera that is 4 cm wide. Find the magnification and the size of the image.

Object size o = 6 cm, u = 2 cm, v = 4 cm. Magnification m = v/u = 4/2 = 2. Image size i = o × m = 6 × 2 = 12 cm.

Note: u is the object distance and v the image distance, both measured from the optical instrument.

Speed of Light

Light travels extremely fast: about 3,00,000 km (3 × 10⁸ m s⁻¹) every second in air or vacuum. It travels more slowly when it enters any other medium.

Reflection

Place a lighted candle in front of an opaque object OJ with a screen MN behind it: the screen shows the object's shadow, while we can see the object itself because its surface scatters the light falling on it. If the object has a polished surface like a mirror, we also see the image of the candle. This bouncing back of light from a surface is reflection.

Light thrown back by a smooth, polished surface goes in one definite direction; this is regular reflection. A rough surface throws light back in many directions; this is diffuse reflection. In the setup above we see the object by diffuse reflection and the candle's image by regular reflection. A diagram showing the path of light as rays is a ray diagram.

Fig. 4.13 Reflection of light at a plane mirror MM₁

Terms: the ray that arrives at the mirror (AI) is the incident ray; I, where it strikes, is the point of incidence; the ray that leaves (IB) is the reflected ray; the line IN drawn at 90° to the surface at the point of incidence is the normal. The angle between the incident ray and the normal (∠AIN) is the angle of incidence (i); the angle between the reflected ray and the normal (∠NIB) is the angle of reflection (r).

Laws of Reflection

1. The incident ray, the normal at the point of incidence and the reflected ray all lie in one plane.
2. The angle of reflection equals the angle of incidence: ∠i = ∠r.

Verification of the Laws of Reflection

1. Spread a white sheet on a table and draw a straight line MM' on it; stand a plane mirror upright along this line.
2. Fix two pins P and Q in front of the mirror.
3. Look into the mirror from the side and fix two more pins P' and Q' so that, in line of sight, P', Q' and the images of P and Q appear along one straight line.
4. Remove the mirror and pins. Join Q and P and extend the line to meet the mirror line at O; join Q' and P' and extend it to O as well.
5. Draw the normal ON at O.
6. QP, ON and P'Q' are all on the paper, i.e. in the same plane, which proves the first law. On measuring, ∠QON = ∠Q'ON, which proves the second law.

Fig. 4.14 Verifying the laws of reflection with pins

Example 1. What happens to a ray that falls normally on a plane mirror?
Solution: For normal incidence ∠i = 0, so ∠r = 0 as well, and the ray goes back along the same path in the opposite direction.

Fig. 4.15 A ray falling normally on a plane mirror

Example 2. A ray makes 30° with a plane mirror. Find the angles of incidence and reflection.
Solution: The angle of incidence is measured from the normal, so ∠i = 90° − 30° = 60°, and by the law of reflection ∠r = ∠i = 60°.

Fig. 4.16 A ray making 30° with the mirror

Image Formed by a Plane Mirror

Light spreads from a point source O in all directions. Two of its rays, OA and OB, strike the mirror and are reflected as the diverging rays AC and BD. These reflected rays appear to have come from a point O' behind the mirror, which is where AC and BD meet when extended backwards. O' is the image of O.

Fig. 4.17 Image of a point source in a plane mirror

If the reflected rays really meet, they form a real image, which can be caught on a screen. If they only appear to meet when extended backwards (a diverging beam), the image is virtual and cannot be caught on a screen. A large object is seen in a mirror because every point of it has its own image point.

Characteristics of the Image in a Plane Mirror

1. The image is the same size as the object.
2. The image is as far behind the mirror as the object is in front of it.
3. The image is virtual and erect.
4. The image is laterally inverted.

Lateral Inversion

The right side of an object appears as the left side of its image. Activity: cut the letter P out of thermocol and look at it in a plane mirror; the image shows the letter reversed from left to right.

Fig. 4.18 Lateral inversion of the letter P

Uses of Plane Mirrors

1. As looking glasses.
2. By opticians, to get the required distance for the eye-testing chart.
3. In some shops, to make the display look bigger or deeper.
4. In solar cookers, to reflect sunlight.
5. In kaleidoscopes and periscopes.

Mirrors Placed at an Angle

Two plane mirrors placed at an angle θ to each other form several images of an object between them. The number of images n is

\[ n = \frac{360^\circ}{\theta} \text{ if } \frac{360^\circ}{\theta} \text{ is odd,} \qquad n = \frac{360^\circ}{\theta} - 1 \text{ if } \frac{360^\circ}{\theta} \text{ is even.} \]

The number of images is always odd. For parallel mirrors (θ = 0) the number is infinite.

Kaleidoscope

A kaleidoscope works on multiple reflection between inclined mirrors. Three plane mirrors inclined at 60° to each other are fixed in a cylindrical tube, with a few pieces of coloured glass at one end. Looking in from the other end, the many images of the glass pieces make beautiful patterns.

Periscope

A periscope lets us see objects that are not in the direct line of sight, e.g. a soldier looking over a wall. It is a tube bent twice at right angles with a plane mirror at each bend, each making 45° with the tube. Light from the object enters the upper opening, is reflected by the first mirror (turned through 90°), travels down the tube, is turned through 90° again by the second mirror and goes into the eye. Some periscopes use prisms instead of mirrors, based on total internal reflection (studied later).

Fig. 4.20 A periscope

Spherical Mirrors

A mirror need not be flat. The two sides of a bright spoon act as spherical mirrors and give images unlike those of a plane mirror. A spherical mirror is a part cut from a hollow sphere; the inner face of the piece is concave and its outer face is convex.

Fig. 4.21 A hollow sphere and a piece cut from it

If the outer (convex) face is silvered and the inside is the shining surface, we get a concave mirror. If the inner (concave) face is silvered and the outside is the shining surface, we get a convex mirror.

Fig. 4.22 Concave and convex mirrors

Terms Used with Spherical Mirrors

Pole (P): the centre of the mirror surface.
Centre of curvature (C) and radius of curvature (R): the centre of the sphere of which the mirror is a part is the centre of curvature, and the radius of that sphere is the radius of curvature. C is in front of a concave mirror and behind a convex mirror.

Fig. 4.23 Pole and centre of curvature

Principal axis: the straight line through the pole and the centre of curvature.
Principal focus (F): take a ray AB parallel to the principal axis. The line BC (C is the centre of curvature) is the normal at B, and the reflected ray BF is drawn so that ∠ABC = ∠CBF. Other rays parallel to the axis, such as KL, GH and IJ, also pass through the same point F after reflection from a concave mirror. This point F on the principal axis is the principal focus of the concave mirror: the point where a beam parallel to the principal axis meets after reflection.

Fig. 4.24 Principal focus of a concave mirror

For a convex mirror, the principal focus is the point on the axis from which a parallel beam appears to diverge after reflection.

Fig. 4.25 Principal focus of a convex mirror

Focal length (f): the distance from the pole to the principal focus. For a small mirror,

\[ f = \frac{R}{2} \]

Example. A concave mirror has radius of curvature 30 cm. Its focal length f = R/2 = 30/2 = 15 cm.

Constructing Ray Diagrams

To find where an image forms, use any two of these three rays.

1. A ray parallel to the principal axis passes through the principal focus after reflection (concave mirror), or appears to come from the principal focus (convex mirror).

Fig. 4.26 Ray parallel to the principal axis

2. A ray that passes through the principal focus (concave) or is directed towards it (convex) goes parallel to the principal axis after reflection.

Fig. 4.27 Ray through the principal focus

3. A line joining the centre of curvature to any point of the mirror is a normal there. So a ray passing through (or directed towards) the centre of curvature falls normally on the mirror and comes back along its own path.

Fig. 4.28 Ray through the centre of curvature

Images Formed by a Concave Mirror

Images Formed by a Convex Mirror

Wherever the object is placed, a convex mirror always gives an erect, virtual and diminished image lying between the pole and the principal focus. For an object at infinity the image is highly diminished and formed at the principal focus.

Fig. 4.29 Images formed by a convex mirror

Uses of Concave Mirrors

1. In torches, headlights and searchlights, as reflectors that send light to a long distance.
2. By dentists, to see the back of the teeth.
3. As shaving mirrors.

Uses of Convex Mirrors

1. As side-view (rear-view) mirrors in vehicles, because they give a wide field of view and a diminished image.
2. As reflectors in street lights.

Important Points about Images in Spherical Mirrors

1. Only a concave mirror can form a real image.
2. All mirrors (plane, concave and convex) can form virtual images. A plane mirror's virtual image is the same size as the object, a concave mirror's virtual image is always magnified, and a convex mirror's image is always diminished.
3. Real images are always inverted; virtual images are always erect.
4. As an object is moved towards a concave mirror (from beyond C up to F) the real image moves away from the mirror. When the object is at C, the image is at C and of the same size.

Light and Colours

Take a circular cardboard disc divided into seven equal sectors coloured violet, indigo, blue, green, yellow, orange and red, with a hole at the centre by which it is fixed on a rod. When the disc is turned slowly the separate colours can be seen, but when it spins fast the whole disc looks white. This disc, first used by Sir Isaac Newton, is Newton's disc.

Fig. 4.30 Newton's disc

A prism is a transparent solid bounded by two triangular faces and three rectangular faces. When a narrow beam of white light passes through a prism, a band of seven colours (violet, indigo, blue, green, yellow, orange, red) is seen on a screen placed behind it.

Fig. 4.31 Dispersion of white light by a prism

So seven colours combine to make white light, and white light can be split into its seven colours. This splitting of white light into its colours is called dispersion of light. Red light is bent the least and violet the most.

Rainbow

A rainbow forms when sunlight is dispersed into its seven colours while passing through raindrops. The Sun is behind the observer, so a rainbow is seen in the direction opposite to the Sun.

Light and the Growth of Plants

Plants need light to grow. The green colour of leaves comes from chlorophyll, which changes light energy into chemical energy in photosynthesis: carbon dioxide from the air and water combine to form glucose, and oxygen is released. This chemical energy is stored in the plant as food. Different colours of light affect plant growth differently.

Intensity of Light

Objects differ in brightness because the amount of light reaching our eyes differs, i.e. because the intensity of light differs. The intensity of a source is found by comparing the light it gives out per unit area in a given time with that of a standard source, usually a standard candle, and is stated in candle power. A source of 50 candle power is 50 times as bright as the standard candle. The SI unit of luminous intensity is the candela. The lumen is another unit, and lux is derived from the lumen.

1 lumen = 12.56 candle power = 1/683 watt, i.e. 1 watt = 683 lumen.
The intensity of light at a place is measured by the lumen falling on unit area; the unit is the foot candle, and the modern unit is the lux: 1 foot candle = 10.76 lux. The intensity of light on a surface decreases as its distance from the source increases.

Points to Remember

1. Light is energy that causes the sensation of vision. Its speed in air is 3 × 10⁸ m s⁻¹. Anything through which light passes is an optical medium.
2. Shadows and eclipses arise because light travels in straight lines.
3. Laws of reflection: (i) angle of incidence = angle of reflection; (ii) the incident ray, reflected ray and normal lie in one plane.
4. An image that can be taken on a screen is real; one that cannot is virtual. Real images are inverted and virtual images are erect.
5. A plane mirror always gives a virtual, erect image of the same size, laterally inverted.
6. For small spherical mirrors, focal length = radius of curvature ÷ 2.
7. Two plane mirrors at angle θ give 360°/θ images (if this is odd) or 360°/θ − 1 images (if even).
8. A convex mirror always gives a virtual, erect, diminished image and is used as a rear-view mirror.
9. A concave mirror gives a virtual, magnified, erect image when the object is between the pole and the focus; in all other positions the image is real and inverted (smaller, equal or larger depending on the object position). Concave mirrors are used in torches, searchlights, shaving mirrors and by dentists; convex mirrors in street-light reflectors and as rear-view mirrors.
10. Splitting of white light into seven colours is dispersion; a rainbow is due to dispersion in raindrops.
11. Intensity of light is compared with a standard candle (candle power); 1 lumen = 12.56 candle power and 683 lumen = 1 watt. Intensity at a place is measured in foot candle (1 foot candle = 10.76 lux).