1. How will you ascertain whether a mirror is concave, convex, or plane without touching it?

We can determine the nature of the mirror by observing the image of an object (a pencil or your finger) placed very close to the mirror. Images produced by all the three mirrors are virtual but the magnifications of the images in this three cases are different. For a plane mirror the image is of same size as the object. A convex mirror produces a diminished image while the image produced by a concave mirror is magnified.

2. Cinema screens are made rough and white – why?

As the surface of the screen is rough, light incident on it undergoes diffused reflection making the image visible from all the portions of the audience. If it was very smooth, reflected light would reach only a small part of the audience. Then too the image wouldn’t be visible due to its excessive brightness.

A white surface reflects all colors equally producing the image in its actual color. A colored screen would change the color of the image.

3. At night it is difficult to see through a closed glass-window from inside of an illuminated room. But this becomes relatively easy if the lights in the room are switched off. – Why?

This happens due to change in relative brightness of the outside objects with respect to other bodies. If the lights are on, the objects in the room are much brighter than the outside objects and the glass also reflects much light making it difficult to see the outside objects. When the room-lights are off – the amount of light reflected by the glass or the other objects in the room decreases considerably making the outside body relatively brighter.

This is similar to the case of stars not being visible in presence of the sun while they become visible in the night-time.

4. A concave mirror forms a real image of an object. If the whole system is immersed in water will there be any change in the distance or size of the image?

No.

The light rays follow the same rules of reflection and thus follow the same paths even inside water. Therefore the image is produced at the same location with respect to the mirror.

5. Light from a filament bulb is reflected by a glass slab. Two images are seen – one is white and the other is red. How can that happen? – explain.

This can happen if the color of the glass slab is red. The light rays incident on the front surface of the slab are partially reflected carrying all the colors emitted by the bulb producing an image that is white in color. The other part of the incident rays enter the glass slab and are reflected by the second surface producing the second image. As these rays travel through the glass the colors other than red are striped off and the produced image appears to be red.

6. When light of wavelength enters water its wavelength changes to where is the refractive index of water. As the wavelength decreases in this case. Does this imply that a diver cannot see the objects inside water in their original colors?

The diver sees them in their correct color.

The color of an object depends on the frequency of light it reflects or emits. Though the light rays of a particular color has different wavelengths in air and water, the frequency remains unchanged and so the diver can see the objects in their actual color.

7. Can a convex lens behave as a diverging lens?

In general the lenses are made up of glass and the surrounding medium is air. Under such conditions a convex lens behaves as a converging lens. In this situation the refractive index of the lens medium is larger than the refractive index of its surrounding. But if we immerse the lens in a fluid such that the refractive index of the fluid is lager than that of the lens then the nature of refraction is opposite and it becomes a diverging lens.

8. A piece of glass disappears when immersed in glycerine – why?

Refractive index of glass is nearly equal to the refractive index of glycerine. So, when rays of light traveling through glycerine are incident on the surface of glass they travel through it uninterrupted without suffering any reflection or refraction. Therefore optically, the glass piece becomes the part of glycerine and does not remain visible.

9. A droplet of water on lotus leaf or arum leaf appears shinning – why?

This happens due to total internal reflection of light. Water droplets cannot make the surface of the leaf wet as a thin layer of air forms beneath it. When a ray of light passes through water (of the droplet) and is incident on the air layer at an angle greater than the critical angle the light suffers total internal reflection giving it a shinning appearance.

10. If two different colors of light are used to determine the refractive index of glass, will the values be same? Explain.

The refractive index of a medium depends on the speed of light in that medium. In a medium, the speed of light for different colors are different. So, the values of refractive indices, determined using different colors of light, are different.

11. Compare the speeds of light of red color and blue color in vacuum. How will be this comparison, if the medium is water?

Speed of light of in vacuum does not depend on its color. So, light of red and blue colors travel at same speed there. But this is not true for any other optical medium. The refractive index of a medium is higher for the blue color than red (i.e. ) in water. Now the refractive index of a medium is given by

   

As is a constant, the refractive index is inversely proportional to the speed of light. As the blue light has a greater value of refractive index, this is slower than the red color in water.

12. Why does the Sun appear red during sunrise?

The Sun rises near the horizon. Therefore, the light travels the longest distance through the atmosphere. While passing through the atmosphere, the blue colors of shorter wavelength are scattered most and lost from the direction of travel. Red color having the longer wavelength are scattered least and the loss of red light is minimum. Therefore, an observes receives the red color more than any other color. Thus the Sun appears red during the sunrise.

13. A ray of light passes from water to glass. How does the speed of light change?

The refractive index of a medium is given by

   

As is a constant, the refractive index of a medium is inversely proportional to the speed of light in it. As the refractive index of glass is higher than that in water (), the speed of light is greater in water. So, we observe a decrease in speed of light as it travels from water to glass.

14. Compare the speeds of light of red and blue colors.

In vacuum all the colors travel at same speed. So, red and blue colors also travel at same speed (= 3×10⁸ m/s) in vacuum. But in any other medium the light of red color travels faster than the light of blue color. Therefore, the refractive index of any medium for blue color is larger than that for red.  

15. Name some phenomena of physics that show the wave properties of light.

In case of interference, diffraction, polarization etc. light exhibits its wave properties. 

16. Name some phenomena of physics that show the particle properties of light.

In case of photoelectricity, Compton effect etc. light exhibits its particle nature.