CBSE Class 7 Science Advanced Physics Light MCQs Set 01

Answer: (c) Mirror

Explanation:
1. Water, glass and a prism let light pass through them, so they are transparent.
2. A mirror has a silvered backing that stops light from going through; it only reflects it.
3. So mirror is the opaque object.

Teacher's Note:
1. Ask: does light pass through the material?
2. A mirror shines because it reflects, not because it transmits.

Answer: (b) solar eclipse

Explanation:
1. The Moon's shadow falling on the Earth means the Moon is between the Sun and the Earth.
2. That arrangement gives a solar eclipse.

Teacher's Note:
1. Shadow of Moon on Earth: solar eclipse.
2. Shadow of Earth on Moon: lunar eclipse.

Answer: (d) 30°

Explanation:
1. The mirror is horizontal, so the normal to it is vertical.
2. The ray makes 30° with the normal, so the angle of incidence is 30°.
3. By the law of reflection the angle of reflection is also 30°, so the reflected ray makes 30° with the normal.

Teacher's Note:
1. Angles of incidence and reflection are always measured from the normal.
2. Here the vertical itself is the normal.

Answer: (d) All the above

Explanation:
1. A plane mirror turns light by regular reflection.
2. A piece of paper and a stone have rough surfaces and send light in many directions by diffuse reflection.
3. So all three change the direction of light: all the above.

Teacher's Note:
1. Any surface that reflects or scatters light changes its direction.
2. Smooth surfaces give regular reflection; rough ones give diffuse reflection.

Answer: (c) equal to the size of the object.

Explanation:
1. A plane mirror forms an image of the same size as the object, at the same distance behind it.
2. So the image is equal in size to the object.

Teacher's Note:
1. Remember the three equalities: size, distance, and (reversed) sides.
2. Only the sides get interchanged, not the size.

Answer: (d) Both (a) and (b)

Explanation:
1. The outer surface of a spoon bulges outwards, so it acts as a convex mirror.
2. A convex mirror always gives an image that is virtual and erect.
3. So the answer is both (a) and (b).

Teacher's Note:
1. Outer face of spoon: convex mirror. Inner face: concave mirror.
2. A convex mirror's image is also diminished.

Answer: (b) concave mirror

Explanation:
1. A plane mirror's image is the same size and a convex mirror's image is smaller.
2. Only a concave mirror, with the object between its pole and focus, gives a virtual image that is magnified.
3. So it is a concave mirror.

Teacher's Note:
1. Magnified and virtual together point to a concave mirror.
2. The object must lie between P and F.

Answer: (b) the deviation of violet light is maximum.

Explanation:
1. A prism bends different colours by different amounts.
2. Red is bent the least and violet the most.
3. So the deviation of violet light is maximum.

Teacher's Note:
1. Order of bending from least to most: red, orange, yellow, green, blue, indigo, violet.
2. This unequal bending is what causes dispersion.

Answer: (d) All the above

Explanation:
1. A convex mirror always forms an image that is virtual, erect and diminished.
2. So all the above are correct.

Teacher's Note:
1. Convex mirror: virtual, erect, diminished, for every position of the object.
2. This is why it gives a wide field of view.

Answer: (c) lux

Explanation:
1. Candle power is the old way of comparing a source with a standard candle.
2. The intensity of light at a place (light falling on unit area) used to be measured in foot candles; its modern unit is the lux (1 foot candle = 10.76 lux).
3. Lumen measures the total light given out, not the intensity at a place.

Teacher's Note:
1. Foot candle is the older unit; lux is the modern one for light on a surface.
2. 1 foot candle = 10.76 lux.

Answer: (c) Both A and B are true

Explanation:
1. Planets shine only by reflecting sunlight, so they are non-luminous. A is true.
2. The Sun is the star nearest to us and gives the Earth most of its light. B is true.
3. So both A and B are true.

Teacher's Note:
1. Check each statement on its own.
2. Stars are luminous; planets and the Moon are not.

Answer: (d) Both (b) and (c)

Explanation:
1. Planets only reflect sunlight and give no light of their own.
2. A glow-worm and the stars produce their own light naturally.
3. So the answer is both (b) and (c).

Teacher's Note:
1. A natural source makes its own light and is not man-made.
2. Bioluminescence is natural light from living things.

Answer: (c) Both A and B are true

Explanation:
1. Light from a single point is blocked entirely along each line, so the shadow has no partly lit border. A is true.
2. Different points of an extended source light up different parts of the screen, giving a dark umbra and a partly dark penumbra. B is true.
3. So both A and B are true.

Teacher's Note:
1. Penumbra needs an extended source.
2. A point source gives a sharp shadow.

Answer: (a) the principal axis

Explanation:
1. The straight line through the pole P and the centre of curvature C is the principal axis of the mirror.

Teacher's Note:
1. The principal focus also lies on this line.
2. Distances along it are measured from the pole.

Answer: (b) Concave mirror

Explanation:
1. A concave mirror brings a parallel beam together at its focus, so it is a converging mirror.
2. A convex mirror spreads the beam out and a plane mirror keeps it parallel.
3. So it is the concave mirror.

Teacher's Note:
1. Concave: converging. Convex: diverging.
2. A plane mirror neither converges nor diverges.

Answer: (d) Both (b) and (c)

Explanation:
1. A convex mirror forms its image between the pole and the principal focus.
2. The image is at F only when the object is at infinity; for every other position it is nearer than F.
3. So the image distance is less than or equal to the focal length: both (b) and (c).

Teacher's Note:
1. Picture the image lying between P and F.
2. At infinity the image reaches F, but never goes beyond it.

Answer: (d) All the above

Explanation:
1. Statement (a) is the second law of reflection.
2. Statement (b) is the first law of reflection.
3. Statement (c) is also true: the reflected ray goes to the other side of the normal.
4. So all the above are correct.

Teacher's Note:
1. Learn both laws word for word.
2. The incident and reflected rays are always on opposite sides of the normal.

Answer: (b) 12.56

Explanation:
1. As given in the chapter, 1 lumen = 12.56 candle power.
2. 10.76 is the number of lux in one foot candle, not the lumen figure.

Teacher's Note:
1. Do not mix 12.56 (lumen) with 10.76 (foot candle to lux).
2. Also remember 1 watt = 683 lumen.

Answer: (a) ACBD

Explanation:
1. First decide whether the image is real or virtual (A).
2. If it is real, the mirror is concave (C) and we stop.
3. If it is virtual, check its size (B).
4. Then conclude: magnified means concave, diminished means convex, same size means plane (D).
5. The order is ACBD.

Teacher's Note:
1. Real image: concave mirror straight away.
2. Only for a virtual image do you need to compare sizes.

Answer: (c) DECBAF

Explanation:
1. Set up first: draw the line and fix the mirror (D), then fix pins P and Q (E).
2. View the images and fix P₁, Q₁ (C).
3. Remove the mirror and pins and join the points (B).
4. Draw the normal (A) and finally measure and compare the angles (F).
5. The order is DECBAF.

Teacher's Note:
1. Think of the experiment as set up, observe, draw, measure.
2. The normal can only be drawn after the lines have met.

Answer: (a) intensity

Explanation:
1. Brightness depends on how much light reaches the eye.
2. The amount of light is described by its intensity.

Teacher's Note:
1. Brighter object means more light reaches the eye.
2. Intensity is the word for that amount.

Answer: (a) at C

Explanation:
1. Object and image are at the same place only when both are at the centre of curvature C.
2. In that case the image is real, inverted and as big as the object.
3. So the object is at C.

Teacher's Note:
1. Remember: at C, u = v = R.
2. The image at C is inverted, so it does not hide the object but sits at the same distance.

Answer: (c) at F

Explanation:
1. Rays from an object at F become parallel after reflection from a concave mirror.
2. Parallel rays meet only at infinity, so the image is at infinity.
3. So the object is at F.

Teacher's Note:
1. Rays through F go out parallel.
2. The reverse is also true: rays from infinity meet at F.

Answer: (a) concave

Explanation:
1. Plane mirrors give same-size images and convex mirrors give smaller ones.
2. A concave mirror can give magnified images (object between C and F, at F or between F and P).
3. So the mirror is concave. Centre of curvature is a point, not a mirror.

Teacher's Note:
1. Only concave mirrors can magnify.
2. Option (d) is a point, not a type of mirror.

Answer: (a) pole

Explanation:
1. The middle point of the reflecting surface of a spherical mirror is its pole (P).

Teacher's Note:
1. Pole = centre of the mirror surface.
2. Centre of curvature = centre of the sphere.

Answer: (b) retraces its path

Explanation:
1. The line from C to any point of the mirror is a normal.
2. A ray along a normal strikes the mirror at 0° and is reflected straight back.
3. So the ray retraces its path.

Teacher's Note:
1. Normal incidence: angle 0°, so it goes back along the same line.
2. Do not confuse this with the ray through F.

Answer: (b) irregular reflection

Explanation:
1. We see non-luminous objects because their rough surfaces send light in all directions into our eyes.
2. This scattering is irregular (diffuse) reflection, so the answer is irregular reflection.

Teacher's Note:
1. Diffuse reflection lets us see an object from any side.
2. Smooth mirrors give regular reflection instead.

Answer: (d) All the above

Explanation:
1. A plane mirror's image is as far behind as the object is in front, same in size and virtual.
2. So all the above are true.

Teacher's Note:
1. Three facts: equal distance, equal size, virtual.
2. Add: erect and laterally inverted.

Answer: (b) infinite

Explanation:
1. Each image acts as an object for the other mirror, and this goes on forever.
2. Using n = 360°/θ − 1 with θ = 0° gives an infinite number.
3. So infinite images are formed.

Teacher's Note:
1. Think of the endless corridor of reflections in two facing mirrors.
2. The formula gives n → ∞ when θ → 0.

Answer: (b) decreases

Explanation:
1. Light spreads over a larger area as it goes farther from the source.
2. So less light falls on each unit area: the intensity decreases.

Teacher's Note:
1. The farther the lamp, the dimmer the light.
2. The same light spreads over a bigger area.

Answer: (a) virtual

Explanation:
1. The image cannot be caught on a screen, so it is virtual.
2. It is erect, same size and laterally inverted, not real, inverted or magnified.

Teacher's Note:
1. Plane mirror: virtual, erect, same size.
2. Real images can be caught on a screen.

Answer: (b) Only B is true

Explanation:
1. The Moon only reflects sunlight, so it is non-luminous. A is false.
2. A glow-worm makes its own light, a natural source. B is true.
3. So only B is true.

Teacher's Note:
1. The Moon shines because of the Sun.
2. Glow-worm light is bioluminescence.

Answer: (d) All the above

Explanation:
1. All three lamps are made by people, so all the above are man-made sources.

Teacher's Note:
1. Man-made sources are called artificial sources.
2. Natural ones are the Sun, stars and glow-worms.

Answer: (c) Both A and B are true

Explanation:
1. Shadows form because light travels in straight lines. A is true.
2. With the Earth between the Sun and the Moon, the Earth's shadow falls on the Moon: lunar eclipse. B is true.
3. So both A and B are true.

Teacher's Note:
1. Both statements are standard facts.
2. Order for a lunar eclipse: Sun, Earth, Moon.

Answer: (d) Both (a) and (b)

Explanation:
1. Light passes through the hole along straight lines and actually meets on the screen, so the image is real.
2. Rays cross at the hole, so the image is inverted.
3. The shape of the hole does not change the nature of the image.
4. So the answer is both (a) and (b).

Teacher's Note:
1. Pinhole image: real and inverted.
2. A smaller hole only makes the image sharper but dimmer.

Answer: (c) radius of curvature

Explanation:
1. C is the centre of the sphere, so the distance from C to any point of the surface is the radius of the sphere.
2. This is the radius of curvature.

Teacher's Note:
1. R = CP = 2f.
2. The principal axis is a line, not a length.

Answer: (d) All of the above

Explanation:
1. A concave mirror gives real images for objects beyond F, and a virtual one for objects between P and F.
2. The image can be diminished, equal or magnified depending on the position.
3. So all of the above are possible.

Teacher's Note:
1. List the object positions and check each case.
2. Only the convex and plane mirrors have fixed image types.

Answer: (d) All the above

Explanation:
1. The principal axis passes through C, so the ray falls along a normal.
2. So the angle with the normal is 0°, the ray passes through C and goes back along its own path.
3. All three statements are true: all the above.

Teacher's Note:
1. The axis is itself a normal at the pole.
2. Zero angle of incidence means zero angle of reflection.

Answer: (c) convex

Explanation:
1. Convex mirrors give a wide field of view and a diminished, erect image, so they are used as rear-view mirrors.
2. The answer is convex.

Teacher's Note:
1. Wide view, small erect image: convex.
2. Plane mirrors have a narrow view.

Answer: (d) All the above

Explanation:
1. Each statement is a correct fact about light, so all the above is the answer.

Teacher's Note:
1. Read every option before choosing 'all of the above'.
2. Infrared radiation gives the feeling of heat.