Question

Optics Conceptual Analysis

One or two sentences is fine. But please clearly explain each question.

For each statement, decide whether it is always true, always false or true in certain
circumstances. State which case applies to the statement, describing why you
chose the answer you did and cite an experiment or principle of Physics which
supports your claim.

[1] Mirrors can only produce virtual images.
[2] A lens which is thicker in the center than the rim will have a positive focal length when
surrounded by media with lower index of refraction.
[3] A point source of light is placed at the focal point of a convex lens. The light emerging from
the other side of the lens is a parallel beam of light.
[4] An object and screen are 1 meter apart. A convex lens with a focal length less than 25cm
will form an image on the screen when placed at either of 2 positions.
[5] Real images are formed by convex lenses and mirrors.
[6] An image an observer can see is a real image.
[7] All concave lenses are divergent lenses.
[8] The thin lens equation only works for visible light, not UV or IR.
[9] The index of refraction for a material is the same for all wavelengths.
[10] For total internal reflection, the index of refraction outside the material must be greater than
that of the material in which the light is travelling.

Answer 1

1) Always False (Concave mirrors can produce real images, so the fact that they said ONLY makes this always false)

2) Always True (By applying the lens makers equation 1/f = (n_lens-n_o)/n_o)(1/R₁ - 1/R₂) shows this will always yield a positive result)

3) Always True (By definition of the focal point, as well as by experiment with ray diagrams)

4) Always True (By applying the lens formula (1/f = 1/p + 1/q) shows that p and q - which are object and image distances - can be interchanged)

5) True in certain circumstances - (Convex mirrors will always produce virtual images and convex lenses will form real or virtual images depending on the location of the object in reference to the focal point)

6) True in certain circumstances - (Both real and virtual images can be seen. Virtual images are images of yourself in front of your bathroom mirror, you can see them. Real images are images on a movie screen, you can see them. So you can see both real and virtual images, not just real ones)

7) True in certain circumstances - (It is possible to surround a divergent lens with a material that makes the lens converging. The fact that they said ALWAYS makes it partially false)

8) Always False (The equation - which I typed in answer number 4 - does not depend on the type of light, just the location of the object and image. The waves will still refract in the lens)

9) True is cetrain circumstances (Seeing a rainbow through a prism shows that some materials have different indices for different wavelenghts)

10) Always False (The opposite is true. The formula is sin(angle) = n2/n1 so the outside material must be less since the sin function can never be greater than 1)