# Homework Help: Help me out,

1. Feb 14, 2008

### pink_ele

a concave mirror of radius R is filled with water upto a height h
h<R)
ref. index of water is x..where shud an object b placed so tat its image is formed on itself?
ans is R-h/x
plz explain how to solve it completely,it was asked in test.

2. Feb 14, 2008

### mgb_phys

Interesting - it's not obvious.
Without the water the object would have to be at the cente of curvature (twice focal length) to form an image on itself.

Remember light bounces off the mirror at the same angle to the normal it arrived.
Light going into the water bends toward the normal, light leaving bends away.

ps. Put a more informative title on the posting - it will help!

3. Feb 15, 2008

### pink_ele

no
ans is R-h/x

4. Feb 16, 2008

### physixguru

Pick a point on the top of the object and draw two incident rays traveling towards the mirror.

Using a straight edge, accurately draw one ray so that it passes exactly through the focal point on the way to the mirror. Draw the second ray such that it travels exactly parallel to the principal axis. Place arrowheads upon the rays to indicate their direction of travel.

Once these incident rays strike the mirror, reflect them according to the two rules of reflection for concave mirrors.

The ray that passes through the focal point on the way to the mirror will reflect and travel parallel to the principal axis. Use a straight edge to accurately draw its path. The ray which traveled parallel to the principal axis on the way to the mirror will reflect and travel through the focal point. Place arrowheads upon the rays to indicate their direction of travel. Extend the rays past their point of intersection.

Mark the image of the top of the object.

The image point of the top of the object is the point where the two reflected rays intersect. If your were to draw a third pair of incident and reflected rays, then the third reflected ray would also pass through this point. This is merely the point where all light from the top of the object would intersect upon reflecting off the mirror. Of course, the rest of the object has an image as well and it can be found by applying the same three steps to another chosen point.

Repeat the process for the bottom of the object.
The goal of a ray diagram is to determine the location, size, orientation, and type of image which is formed by the concave mirror. Typically, this requires determining where the image of the upper and lower extreme of the object is located and then tracing the entire image. After completing the first three steps, only the image location of the top extreme of the object has been found. Thus, the process must be repeated for the point on the bottom of the object. If the bottom of the object lies upon the principal axis (as it does in this example), then the image of this point will also lie upon the principal axis and be the same distance from the mirror as the image of the top of the object. At this point the entire image can be filled in.

Good luck!!!