## Topic outline

• ### Light

##### Q37. How can you show the image of the sun on a paper?

Ans. Take a concave mirror. Hold it facing the sun. Try to get the light reflected by the mirror on a sheet of paper. Adjust the distance of the paper until you get a sharp bright spot on it. This bright spot is, in fact, the image of the sun.

##### Q38. The rear view mirror of a car is a plane mirror. A driver is reversing his car at a speed of 2 m/s. The driver sees in his rear view mirror the image of a truck parked behind his car. The speed at which the image of the truck appears to approach the driver will be _________.

Ans. The image of the truck will travel a distance twice the distance travelled by the car in equal time. Therefore, image of the truck will appear to approach the driver with the speed of (2 + 2) m/s i.e. 4 m/s.

##### Q39. Write an experiment to show that the sunlight consists of seven colours.

Ans. Take a glass prism. Allow a narrow beam of sunlight through a small hole in the window of a dark room to fall on one face of the prism. Let the light coming out of the other face of the prism fall on a white sheet of paper or on a white wall. We see colours similar to those in a rainbow. This shows that the sunlight consists of seven colours.

##### Q40. Why the word ambulance is written laterally inverted on ambulance vehicle?

Ans. The word ambulance is written laterally inverted on ambulance vehicle because when the driver of a vehicle ahead of an ambulance looks in her/his rear view mirror, she/he can read “AMBULANCE” written on it and give way to it. It is the duty of every one of us to allow an ambulance to pass without blocking its way.

##### Q41. Find out the letters of English alphabet or any other language known to you in which the image formed in a plane mirror appears exactly like the letter itself. Discuss your findings.

Ans. Image of letters of English alphabet such as A, H, I, M, O, T, U, V, W, X, Y formed in a plane mirror appears exactly like the letter itself. Vertical symmetry is found in the letters A, H, I, M, O, T, U, V, W, X and Y. This means that the right side is a reflection of the left.

##### Q42. What is the difference between an object and an image?

Ans. Place a lighted candle in front of a plane mirror. Try to see the flame of the candle in the mirror. It appears as if a similar candle is placed behind the mirror. The candle, which appears behind the mirror, is the image of the candle formed by the mirror. The candle itself is the object.

Image from NCERT

##### Q43. David is observing his image in a plane mirror. The distance between the mirror and his image is 4 m. If he moves 1 m towards the mirror, then find out the distance between David and his image.

Ans. Distance between the mirror and David’s image is 4m.

If the David moves 1 m towards the mirror, then the distance between mirror and David’s image will be (4 – 1) m = 3m

We know that in case of plane mirror image is at the same distance behind the mirror as the object is in front of it. Therefore,

Distance between David and mirror = Distance between mirror and David’s image

So, Distance between David and his image = Distance between David and mirror + Distance between mirror and David’s image (i.e. 3 + 3 = 6 m)

##### Q44. Show that the image is at the same distance behind the plane mirror as the object is in front of it.

Ans. Take a chess board. Draw a thick line in the middle of it. Fix a plane mirror vertically on this line. Place a pencil sharpner, at the boundary of the third square counting from the mirror. Note the position of the image. Now shift the object to the boundary of the fourth square. Again note the position of the image. We will find that the image is at the same distance behind the mirror as the object is in front of it.

Image from NCERT

##### Q45. Show how light gets reflected from a plane mirror with the help of an activity.

Ans. Take a torch. Cover its glass with a chart paper which has three slits. Spread a sheet of chart paper on a smooth wooden board. Fix a plane mirror strip vertically on the chart paper. Now direct the beam of light on the mirror from the torch with slits. Place the torch in such a way that its light is seen along the chart paper on the board. Now adjust its position so that the light from the torch strikes the plane mirror at an angle. We will see that the mirror change the direction of light that falls on it. Now move the torch slightly to either side. We will see change in the direction of reflected light too. Look into the mirror along the direction of the reflected light. We see the slits in the mirror. This is the image of the slits. This activity shows how light gets reflected from a plane mirror.

Image from NCERT

##### Q46. State the differences between a convex and a concave lens.

Ans. Difference between a convex and a concave lens

 Convex Lens Concave Lens 1. Those lenses which feel thicker in the middle than at the edges are convex lenses. 1. Those lenses which feel thinner in the middle than at the edges are concave lenses. 2. A convex lens converges (bends inward) the light generally falling on it. Therefore, it is called a converging lens. Image from NCERT 2. A concave lens diverges (bends outward) the light and is called a diverging lens. Image from NCERT 3. A convex lens can form real and inverted image. When the object is placed very close to the lens, the image formed is virtual, erect and magnified. 3. A concave lens always forms erect, virtual and smaller image than the object.