# The time span for which insect can see its image?

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1. Feb 7, 2015

### sylwesh98

1. The problem: A plane mirror of length 2m is kept along the line y=-x as shown in the figure. An insect having velocity of 4 cm/s is moving along x-axis from far away. The time span for which the insect can see its image will be:

A)50 sec B)25 sec C)25√2 sec D)50√2 sec

2.The answer given is D-50√2 sec

3. The attempt at a solution: I've tried to solve it by using laws of reflection. but i cant get the answer that is given from the options. So please help!

My solution was : the insect can see its image form the bottom of the mirror ie., for 2m for x-axis . so the time span is 2m=200 cm
time span= 200/4=50sec
but why is the answer 50√2 .
Have i done anything wrong?

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2. Feb 7, 2015

### Bystander

How did you use the laws of reflection? Remember, the insect has to be able to see its own image.

3. Feb 7, 2015

### sylwesh98

yah to see its own image, i thought it has to be below the mirror.

4. Feb 7, 2015

### Bystander

Where below the mirror?

5. Feb 7, 2015

### sylwesh98

Yah! I thought so, but i dont know for where it can see its image.

6. Feb 7, 2015

### Bystander

What's the angle of incidence for light from the insect to strike the mirror and return to the insect's eye(s)?

7. Feb 7, 2015

### sylwesh98

can you please say from where ant is able to see its own image? so that i can try the question again!

8. Feb 7, 2015

### sylwesh98

I think it should be 90 degrees so that is see its own image

9. Feb 7, 2015

### Bystander

Very good. Where on the x-axis can you arrange 90 degree incident angles with the mirror?

10. Feb 7, 2015

### sylwesh98

i did a small mistake now i get it! Thanks for the help!

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