Light, Shadows and Reflections

Q38. List the three conditions necessary for formation of shadow.

Ans. The three conditions necessary for formation of shadow are:

      1.   There must be a source of light.

      2.   There must be an opaque object to obstruct the light.

      3.   There must be an opaque screen where shadow will be formed.

Q39. Differentiate between luminous and non-luminous objects.

Ans.

Q40. How a shadow differs from an image?

Ans.  Difference between a shadow and an image

Q41. Explain reflection of light with help of an activity.

Ans. This activity should be done at night or in a dark room. Ask one of your friends to hold a mirror in his/her hand at one corner of the room. Stand at another corner with a torch in your hand. Cover the glass of torch with your fingers and switch it on. Adjust your fingers with a small gap between them so that you can get a beam of light. Direct the beam of the torch light onto the mirror that your friend is holding. Now, adjust the direction of the torch so that the patch of light falls on another friend standing in the room. This shows light travel along straight lines and gets reflected from a mirror.

Q42. Differentiate between transparent, translucent and opaque object.

Ans.

Transparent Object

      1.   Substances which allows light to pass through them easily, and through which we can see clearly are called transparent objects.

      2.   Example: water, glass, air, cellophane paper etc.

Translucent Object

      1.   Substances through which light can pass partially but we cannot see through them clearly are called translucent objects.

      2.   Example: butter paper, greased paper, frosted glass, wax paper etc.

Opaque Object

      1.   Substances which do not allow light to pass through them at all are called opaque object.

      2.   Example: stone, metals, bricks, plastic etc.

Q43. Explain with an example that light travels in straight line.

Ans. Light travels in straight line can be demonstrated by the following experiment:

Take three cardboards and make a hole at their centers. Place a burning candle at one end of the row of cardboards and arrange all the three cardboards in such a manner that the three holes and the candle flame are in a straight line. The flame of the candle will be visible through the hole of last cardboard arranged in row. Now slightly move any one of the cardboards and try to see the flame again through the hole. The flame will not be visible. From this it is clear that light travels in a straight line.