Class 7 Science Chapter 13 Question Answers - Light

Q1. Differentiate between a real image and a virtual image.
Ans: Difference between a real image and a virtual image is as follows:

Q2. Explain Newton Disc.
Ans: The Newton disc was invented by Isaac Newton. It is a spherical disc containing 7 segments. Each segment corresponds to one colour in the rainbow. Thus, the seven colours present in the Newton disc are violet, indigo, blue, green, yellow, orange and red. When the disc is rotated the colours start to fade to form a single white colour. Newton, thus demonstrated that white light is a combination of 7 different colours found in the rainbow with the help of Newton disc.

Q3. Explain how a concave mirror can be used to burn a piece of paper.
Ans: Concave mirror is also known as converging mirror. When the sunlight passes through the concave mirror and reaches the paper it forms a sharp focus. The paper and the mirror are held in the position for some time and slowly the paper starts burning at the point of focus. It is due to convergence of all the light/heat energy at one point. This is how a concave mirror is used to burn a piece of paper.

Q4. Differentiate between Concave and Convex Lenses.
Ans: Difference between Concave and Convex Lenses are as follows:

Q5. Demonstrate that Light Travels in a Straight Line.
Ans: Stepwise process to demonstrate that light moves in a straight line:

1. Light a candle.
2. See the burning candle through a straight pipe.
3. Second time, view the burning candle through a bent pipe.

Result: The first time you viewed the candle through a straight pipe light reached your eyes whereas in the second attempt you could not see the light.

Q6. Explain the Formation of a Rainbow.
Ans: A rainbow is a spectrum of light consisting of seven different colours. We usually see a rainbow after rainfall. It is formed when the sunlight falls on the water droplets present in the sky after rainfall. The raindrops act as a prism and split the sunlight in seven different colours i.e. violet, indigo, blue, green, yellow, orange and red. Formation of the rainbow involves reflection, refraction and dispersion of light.


Q7. List a Few Uses of Spherical Mirrors.
Ans: Spherical mirrors are of two types: i) Concave Mirror, ii) Convex Mirror.
Uses of Concave Mirror:

• It is used in solar cookers.
• It is used by dentists.
• It is used in torches, head light of vehicles as reflector.
• It is used as shaving mirrors to see enlarged faces.

Uses of Convex Mirror:

• It is used as a rear view mirror in vehicles.
• It is also used in security mirrors at any place.