Another physics question..?

Question

A being for a distand galaxy is 2m tall and has one eye situated exactly at the top of its head. What is the least length of mirror that it needs in oreder to see itself fully in the mirror. Show with a ray diagram that it does not depend on how far the mirror is held from the alien being....

i understand that you cant draw the ray diagram...but this question just stumps me completely..!!

Answer 1

Is this a trick...

his eye is at the top of its head so how can he see himself directly I mean he can see his head but the rest of his 2m body?

Answer 2

ahhh this is similar to a question on the physics specimen paper for oxford entry.

basically, you have to think about how the light will travel to get to his eye - so that he can see the image.

for him to see the top of his head the light would just reflect straight off of his head to the mirror and back again. so the minimum height the top of the mirror needs to be is 2m.

for him to see his feet, the light needs to bounce off of his feet and travel upwards at an angle to the mirror and then to his eyes. the light would always hit the mirror at half his height for it to be reflected back to his eye.

so the height the bottom of the mirror needs to be is 1m.

so the minimum length of the mirror for him to be able to see all of himself is 1m - half his height.

i hope i've explained this alright! it would be easier with a diagram.

hope this helps =]