Class7 light

                                   Class 7  Science

Lesson-15      Light and Associated Phenomena

I. Answer the following questions in brief.

1. What is reflection of light?

Ans.

      When light falls on an object, it changes its direction and bounces back. This bouncing back of lightis known as reflection of light

2. Define the following terms:

    a) Incident ray      b) Reflected ray      c) Principal focus     d) Normal        e) Point of incidence

Ans.

       a) Incident ray- the ray of light that falls on the surface of the reflecting material or mirror.

       b) Reflected ray- the ray of light that is sent back from the surface of the reflecting surface.

       c) Principal focus- all the rays of light reflected from the curved surface of a concave mirror meet at

          a point. This point is called the principal focus.

       d) Normal- the perpendicular line drawn from the point of incidence on the reflecting surface.

       e) Point of incidence- the point at which the light ray strikes the reflecting surface.

3. What do you mean by lateral inversion?

Ans.

      In the image formed by a plane mirror there is an interchange of left and right. This is called lateral

      inversion.

II. Answer the following questions in detail.

1. What are regular and irregular reflections? Explain with the help of diagrams.

Ans.

      Smooth, shiny surfaces have a regular reflection: After reflection all rays travel in the same direction

      Rough, dull surfaces have a diffused reflection. Diffused reflection scatters light in different directions

      and help us to see things.

2. With the help of an activity, prove that light travels in a straight line.

Ans.

     Take three cardboards A, B and C and make a pinhole at their centers. Place a burning candle on one side of A and arrange the cardboards in such a way that the three pinholes and the candle flame are in a straight line. The candle flame will be visible through the pinhole of the cardboard C.

                            

Now slightly displace any one of the cardboards and try to see the flame through the pinhole of the cardboard C. The flame will not be visible. From this it is clear that light travels in a straight line.