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Question

Fill in the following blanks with suitable words:

(a) The angle of ................... equals the angle of reflection.

(b) A person 1 m in front of a plane mirror seems to be ................... m away from his image.

(c)  If you touch your ................... ear with right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with ...................

(d) The size of pupil becomes ................... when you see in dim light.

(e) Night birds have ...................cones than rods in their eyes.

(f) The image of an object persists on the retina of an eye for about ................... second even after the object has disappeared.

(g) If the still pictures of a moving object are flashed on our eyes at a rate faster than ................... pictures per  second,the eye perceives the object as moving.

(i) In a movie, the still pictures in proper sequence are projected on the screen usually at the rate of ................... pictures per second.


Solution

(a) Incidence 
(The angle of reflection is always equal to the angle of incidence.)

(b) A person 1 m in front of a plane mirror seems to be 2 m away from his image.

(Object distance and image distance are the same from a plane mirror. The image of a person 1 m in front of a mirror is 1 m back to the mirror. Hence, the image is 1 + 1 = 2 m away from the person.)


(c) If you touch your __left__ ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with your _left hand_.

(This is because of lateral inversion of images formed in a plane mirror.)

(d) The size of the pupil becomes __large__ when you see in dim light.

(In the dim light, the amount of light entering the eye is very little. To increase the amount of light, the pupil expands. )


(e) Nightbirds have __less__ cones than rods in their eyes.


(Nightbirds can see in the night, but not in the day. They have on their retina a large number of rod cells and only a few cones.)

(f) The image of an object persists in the retina of an eye for about 1/16th of a second.


This means that, if an object moves with this speed we can see its pictures but if its speed is more than 1/16th of a second, we can see movies or something running with a flow. This is the concept by which we watch the movies.

(g) If the still pictures of a moving object are flashed on our eyes at a rate faster than 16 pictures per second, the eye perceives the object as moving.

(i) In a movie, the still pictures in proper sequence are projected on the screen usually at the rate of 24 pictures per second.


Physics
Science for Eighth Class
Standard VIII

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