CBSE Class 7 Science Advanced Physics Light Very Short Answer Questions Set 03

Very Short Answer Questions

Question 1. What is rectilinear propagation of light?
Answer:
It is the travel of light in a straight line in a homogeneous medium.
In simple words: Light goes straight.

Teacher's Note:
1. Shadows and eclipses prove it.
2. It is also called straight-line travel.

Question 2. What is a lunar eclipse?
Answer:
When the Earth comes between the Sun and the Moon and its shadow falls on the Moon, the Moon is hidden partly or fully. This is a lunar eclipse; it occurs on a full-moon night.
In simple words: The Earth's shadow darkens the Moon.

Teacher's Note:
1. Order: Sun, Earth, Moon.
2. Total or partial.

Question 3. What is reflection?
Answer:
The bouncing back of light from a surface into the same medium is reflection.
In simple words: Light turns back from a surface.

Teacher's Note:
1. It follows the two laws of reflection.
2. Mirrors give regular reflection.

Question 4. What is a real image?
Answer:
An image formed by the actual meeting of reflected or refracted rays is a real image. It can be caught on a screen and is inverted.
In simple words: An image you can put on a screen.

Teacher's Note:
1. A concave mirror can form it.
2. Virtual images cannot be caught on a screen.

Question 5. Write the mathematical expression to calculate the number of images formed by two plane mirrors placed at an angle θ.
Answer:
\( n = \dfrac{360^\circ}{\theta} \) if \( \dfrac{360^\circ}{\theta} \) is odd
\( n = \dfrac{360^\circ}{\theta} - 1 \) if \( \dfrac{360^\circ}{\theta} \) is even
In simple words: Divide 360° by the angle and subtract one when the result is even.

Teacher's Note:
1. Examples: 90° gives 3 images, 60° gives 5.
2. θ = 0° gives infinite images.

Question 6. Define centre of curvature.
Answer:
The centre of the sphere of which the mirror is a part is the centre of curvature (C).
In simple words: The middle point of the original sphere.

Teacher's Note:
1. It is in front of a concave mirror and behind a convex mirror.
2. Radius of curvature R = CP.

Question 7. How many colours does white light consist of and what are they?
Answer:
White light consists of seven colours: violet, indigo, blue, green, yellow, orange and red (VIBGYOR).
In simple words: Seven colours make white light.

Teacher's Note:
1. The mnemonic is VIBGYOR.
2. A prism separates them.

Question 8. What are hot and cold sources of light?
Answer:
A hot source gives light only when it is very hot (Sun, candle, filament bulb). A cold source gives light even when it is cool (glow-worm).
In simple words: Hot sources glow because of heat; cold sources do not.

Teacher's Note:
1. The way of making light decides the type.
2. Cold light is also called bioluminescence in living things.

Question 9. Define umbra and penumbra.
Answer:
Umbra is the completely dark central part of a shadow. Penumbra is the partly dark region around the umbra.
In simple words: Umbra is the full shadow; penumbra the half shadow.

Teacher's Note:
1. Penumbra needs an extended source.
2. They are formed on a screen behind an opaque object.

Question 10. What is a solar eclipse?
Answer:
When the Moon comes between the Sun and the Earth and its shadow falls on the Earth, the Sun is hidden partly or completely. This is a solar eclipse; it happens on a new-moon day.
In simple words: The Moon blocks the Sun.

Teacher's Note:
1. Order: Sun, Moon, Earth.
2. Never look at it with bare eyes.

Question 11. What is diffuse reflection?
Answer:
The scattering of light in many directions from a rough surface is diffuse (irregular) reflection.
In simple words: A rough surface sends light everywhere.

Teacher's Note:
1. It lets us see objects from all sides.
2. A mirror gives regular reflection.

Question 12. What is a virtual image?
Answer:
An image formed where reflected rays only appear to meet when extended backwards is a virtual image. It cannot be caught on a screen and is erect.
In simple words: An image that cannot be put on a screen.

Teacher's Note:
1. Plane and convex mirrors always form it.
2. A concave mirror forms it when the object is between P and F.

Question 13. Define pole.
Answer:
The geometric centre of the reflecting surface of a spherical mirror is its pole (P).
In simple words: The middle point of the mirror.

Teacher's Note:
1. Distances such as f and u are measured from P.
2. P lies on the principal axis.

Question 14. Define principal focus.
Answer:
For a concave mirror, the principal focus F is the point on the principal axis where rays parallel to the axis meet after reflection.
For a convex mirror, it is the point on the axis from which parallel rays appear to diverge after reflection.
In simple words: The meeting point (or apparent source point) for rays parallel to the axis.

Teacher's Note:
1. Concave: real focus in front. Convex: virtual focus behind.
2. f = PF.

Question 15. Write the relationship between focal length and radius of curvature.
Answer:
\( f = \dfrac{R}{2} \), i.e. the focal length is half the radius of curvature, or R = 2f.
In simple words: Focal length is half of the radius.

Teacher's Note:
1. For R = 30 cm, f = 15 cm.
2. This is valid for mirrors with small aperture.

Question 16. What happens to white light when it passes through a prism?
Answer:
It splits into seven colours which spread out as a band on a screen: this is dispersion. Red bends least and violet most.
In simple words: The prism spreads white light into its colours.

Teacher's Note:
1. The band is the spectrum.
2. Rainbow is also dispersion.

Question 17. What is the SI unit of luminous intensity?
Answer:
The SI unit of luminous intensity is the candela.
In simple words: Candela is the unit.

Teacher's Note:
1. Older comparison used 'candle power'.
2. Lumen and lux are related units.