Find the shortest way: A smart crow is on a tree at point ‘A’ as shown in figure-2. Some grains are on the ground. If the crow wants to take a grain and reach the point ‘B’ on the other tree as early as possible(in least time), from where should the crow pick up the grain?
With the mathematical knowledge yo have about angles and triangles can you guess the path that the crow selects? If you can’t, read the following.
The crow can pick the grain from any point on the ground. But the condition is selecting a point on the ground to reach point ‘B’ from point ‘A’ in least possible time. If we assume that the speed of the crow is constant, the path that crow selects should be the shortest. Let us find the shortest path.
Observe some of the paths in the figure-3.
Which among the paths ACB, ADB, AEB and AFB is the shortest path?
To compare the lengths of these paths, make the duplicates of them as shown in
figure-4.
In the figure CB = CG. The length of path ACB = AC+CB = AC+CG = ACG. Thus the length of the path ACG is equal to the length of the path ACB. similarly ,
length of the path ADB = length of the path ADG
length of the path AEB = length of the path AEG
length of the path AFB = length of the path AFG
If you observe the Fig-4 carefully, you will notice that, among the paths ACG, ADG, AEG and AFG the shortest path is AEG, because it is the straight line distance between points A and G. You can measure and check this using a scale. As AEG=AEB, path AEB is the shortest path to reach point B from point A. It would take the least time. So the smart crow will pick the grain from point E.
Observe the path AEB once again in figure-5.
If we draw a normal EEI at point E, we can easily find that angle AEEI (angle 1) is equal to angle EIEB (angle 2).
Like the crow in the above situation, light also selects the path which takes the least time to travel. This principle was first given by Pierre de Fermat, a French
lawyer and an amateur mathematician.
It is also applicable to reflection of light. When light gets reflected from a surface, it selects the path that takes the least time. That is why the angle of incidence is equal to the angle of reflection as shown in figure-5.
Now, before the detailed discussion on reflection, peform a fun activity
and refresh your previous knowledge.
10th Physics New Text Book - Activity 1 from Reflections of Light
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.
Think and discuss
• Does the explanation match with your observation?
• What happens if the hole is much bigger i.e. equal to the size of the flame?
• If it is so, can we get the image of flame on the screen of pinhole camera? Why?
• What happens if we observe the same flame with the same pinhole camera from a far distance?
Think and say. Do the experiment and check your answer.
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.
Think and discuss
• Does the explanation match with your observation?
• What happens if the hole is much bigger i.e. equal to the size of the flame?
• If it is so, can we get the image of flame on the screen of pinhole camera? Why?
• What happens if we observe the same flame with the same pinhole camera from a far distance?
Think and say. Do the experiment and check your answer.
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.
− 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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