Class 7 Science Chapter 13 Question Answers - Light

Q1. We need a shiny surface for reflection. Explain why.
Ans: Since, the extent of reflection depends upon the shine and smoothness of the surface. So, greater the shining and smoothness of the surface, greater will be the reflection. So, this is a reason why we require a shiny surface for reflection.

Q2. Differentiate between a plane mirror, concave mirror and a convex mirror without touching them.
Ans: These mirrors can be differentiated by bringing our face close to each mirror turn by turn. Since, a plane mirror will produce an image of the same size as our face while a concave mirror will produce a magnified image and our face will look much bigger and on other sides, a convex mirror will produce a diminished image and our face will look much smaller like a small child.

Q3. Mention the type of image
(a) which can be formed on the screen.
(b) which cannot be formed on the screen.
Ans:
(a) The image which can be formed on the screen is real image.
(b) The image which cannot be formed on the screen is virtual image.

Q4. Mention any two letters of English alphabet whose image formed in a plane mirror appears exactly similar to the letters.
Ans: Since, image formed by the plane mirror shows lateral inversion, i.e. left seems to right or vice-versa. But in the case of alphabets A and H, these letters show the same image in the plane mirror.

Q5. State the correct sequence (1-7) of colours in the spectrum formed by the prisms A and B shown in the figure.
Ans: When a white light is passed through a prism, it disperses into its seven constituent colours.

Q6. Explain the term lateral inversion.
Ans: When an image is formed by the mirror in such a way that the left side of image is object’s right side and right side of image is object’s left side. So, this process of change in the sides of the object as well as of image is known as lateral inversion.

Q7. State the characteristics of images formed by a concave mirror.
Ans: The characteristics of image formed by a concave mirror are:

• The image formed by a concave mirror is not always the same size as the object. It can be either larger or smaller depending on the distance of the object to the mirror.
• The image formed by a concave mirror is inverted when the object is kept far from the mirror. So, when it is brought very close to the concave mirror, then an erect and enlarged image is formed.

Q8. Explain with the help of an activity that when seven colours are mixed together, then white colour is formed.
Ans: Firstly, take a circular cardboard disc of about 10 cm diameter, then divide this disc into seven equal segments and paint the seven colours of white light (red, orange, yellow, green, blue, indigo and violet) on these segments (as shown in figure given ). Instead of painting the segments, we can also paste seven coloured papers on these segments. So, this disc painted with the seven colours of spectrum is called Newton’s disc. Now, make a small hole at the centre of the disc.
Fix the disc loosely on the tip of a refill of a ball pen. Be confirm that the disc can rotate freely. Now, rotate the disc quickly in day light. So, when the disc is rotated very fast, then all the seven colours mix together due to which the disc appears to be white (see figure). Therefore, the rapid rotation of Newton’s colour disc tells us that mixing of seven colours of the spectrum can give us white light.

Q9. Give the definition dispersion of light. Also, explain why does it take place? Draw a diagram to show the dispersion of white light by a glass prism.
Ans: In the year 1665, Newton discovered by his experiments with glass prisms that white light (like sunlight) consists of a mixture of lights of seven colours. Newton found that if a beam of white light is passed through a glass prism, then the white light splits to form a band of seven colours on a white screen . The band of seven colours formed on a white screen, when a beam of white light is passed through a glass prism, is known as spectrum of white light. The seven colours of the spectrum are Red, Orange, Yellow, Green, Blue, Indigo and Violet.
So, dispersion of light is phenomenon of splitting up of white light into seven colours on passing through a transparent medium like a glass prism. The formation of spectrum of seven colours indicates that white light is a mixture of seven colours. White light can be sunlight. So, now we can say that the sunlight consists of seven colours.
We can mix these colours to get white light. This can be done by using Newton’s disc, let us try this.

Q10. What is regular reflection?
Ans: When parallel rays of light fall on a smooth or polished surface, the reflected rays are parallel to each other. This is called regular reflection.

Q11. What is the kind of image formed by a concave mirror?

Ans: Concave mirror forms real and inverted image but when the object is placed very near to the mirror then the image formed to be virtual erect and magnified.

Q12. List two uses of concave mirror.
Ans: Following are two uses of concave mirror:
(i) It is used by dentists and doctors.
(ii) Concave mirrors are used in head lights of the cars, buses, shaving mirror, etc.

Q13. State any two uses of convex lens.
Ans: (i) It is used as a magnifying glass in hand lenses, microscope, telescope, etc.
(ii) It is used in spectacles to correct far-sightedness.

Q14. What are the characteristics of image formed by a convex mirror?
Ans: Following are the characteristics of image formed by a convex mirror:

• Image is erect.
• The image is smaller than the object.
• The image formed is virtual.

Q15. List two uses of convex mirror.
Ans: Following are the two uses of convex mirror:
(i) It is used as a side view mirror to help the drivers to see the traffic behind them.
(ii) It is used in shops, parking lots, etc., to get larger view of the area.

Q16. What is light?
Ans: Light is a form of electromagnetic radiation (radiation energy) which produces in us the sensation of vision. Light travels along a straight line.

Q17. Differentiate between the following:
(a) Convex mirror and convex lens
(b) Concave mirror and concave lens
(c) Convex mirror and concave mirror
Ans:

Q18. What is meant by spectrum? Name the colours of spectrum in their order.
Ans: The band of seven colours dispersed by a glass prism is known as spectrum. The seven colours in a spectrum are violet, indigo, blue, green, yellow, orange, and red. It is also abbreviated as VIBGYOR.

Q19. What are the characteristics of images formed by a concave mirror?
Ans: Following are the characteristics of images formed by a concave mirror:

• The image is not always of the same size as the object.
• The image is formed on the screen and the type of images which are formed on screen are called real images.
• The image is inclined when the object is kept far away from the mirror. When it is brought very close to the concave mirror, an erect and enlarged image is formed.

Q20. Write a short note on Newton’s disc.
Ans: A Newton disc, invented by Isaac Newton, is a disc with seven segments in rainbow colours. When the disc is rotated fast, the colour get mixed together and the disc appears to be whitish. This is due to the phenomenon called persistence of vision. It was an important discovery as it proves that light is not white, but a mixture of colour. The seven colours used to paint Newton disc are violet, indigo, blue, green, yellow, orange and red.