March 3, 2014

10th Class Physics New Syllabus - Reflections of Light

Reflection of light on different surfaces: In class 6, we have learnt about shadows and we did many experiments with light rays and also discussed the rectilinear propagation of light i.e., light travels in a straight line. In class 7 we learnt the laws of reflection. Let us recall some of them.

− A source of light, an opaque object and a screen are needed to form a shadow.
− Light travels in a straight line.
− When light gets reflected from a surface, the angle of reflection is equal to the angle of incidence.
− The incident ray, the normal at the point of incidence and the reflected ray lie in the same plane.
You must have observed shadows and images in your daily life. You might have got questions in your mind while observing these shadows or images.
• Why does our image appear thin or bulged out in some mirrors?
• Why is there right-left inversion (lateral inversion) when we look in to mirror?
• Can we focus the sunlight at a point using a mirror instead of a magnifying glass?
• Why the angle of reflection is equal to the angle of incidence when a light ray reflects from a surface?
• Are angle of reflection and angle of incidence also equal for reflection on curved surfaces?

In this lesson we are going to learn reflection of light in detail so that we can answer the above questions. Let’s start with some activities based on your previous knowledge.

Activity 1:

Formation of image in pinhole camera

You might have remember, that how an image forms in a pinhole camera that you have learnt in class 6. Draw a ray diagram of formation of an image in pinhole camera.
Think, what happens if we increase the size of the hole of pinhole camera. Observe the flame of a candle with pinhole camera making a big hole to it.

Try to draw a ray diagram for the formation of image in a pinhole camera with a big hole. Look at figure 1. By observing the figure we can understand that the light rays coming from the top of the candle flame fall at different points on the screen. Similarly the rays coming from bottom of the candle flame also fall at different points on the screen. Thus we get blurred image on the screen due to the big hole of the camera as shown figure 1.

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