Thin Lenses. Any lens that is thicker in the center than at the edges will make parallel rays converge to a point and is called a converging lens. Lenses.

Thin Lenses

Any lens that is thicker in the center than at the edges will make parallel rays converge to a point and is called a converging lens.

Lenses that are thinner in the center than at the edges are called diverging lenses because they make parallel light diverge.

Finding the Image Position Formed by a Thin Lens

-ray 1 is drawn parallel to the axis; therefore it is refracted by the lens so that is passes along a line through the focal point;

-ray 2 is drawn on a line passing the other focal point F’ and emerges from the parallel to the axis;

-ray 3 is directed toward the very center of the lens, this ray emerges from the lens at the same angle as it entered.

The Lens Equation

Converging Lens: fdd i

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0

Diverging Lens:

fdd i

111

0

1. The focal length is positive for converging lens and negative for diverging.

2. The object distance is positive if it is on the same side of the lens from where the light is coming; otherwise it is negative.

3. The image distance is positive if it is on the opposite side of the lens from where light is coming.

4. The height of the image is positive if the image is upright and negative if the image is inverted relative to the object.

Lateral Magnification

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1. You find a diverging (converging) lens in the storeroom and wish to define its focal lens. Describe how you could use two lasers to accomplish this.

2. What type of lens would you use to construct an overhead projector? Explain your reasoning.

3. Two converging lenses with identical shapes are made from glasses with different indices of refraction. Which one has the shorter focal length? Why?

4. What type of lens would be helpful in starting of a campfire? Why?

Optical Instruments

Pinhole Cameras

Cameras

Our Eyes

Normal Eye

Lens relaxed, focused at infinity:

Lens thickened, focused on nearby object:

Correcting eye defects with lenses

Test for Astigmatism

The Magnifying Glass

Telescopes

Newtonian Cassegrainian

1. In a refracting telescope, parallel light is collected from a distant source to form a real image that is then magnified by the eyepiece. To achieve high magnification, this real image in located near the eyepiece’s focal point. Use this information to determine how the length of a refracting telescope compares with the local lengths of the lenses.

2. Sometimes when a person develops a cataract, the lens of the eye is surgically removed and replaced with a plastic one. Would you expect this lens to be converging or diverging one? Explain.

Baby-Quiz1. Why is the diamond more brilliant than a clear piece of glass

having the same shape?

2. A surface appear yellow under the white light. How it will appear under red light? Under green light? Under blue light?

3. Most of us find that we really have to strain our eyes to focus on objects located close to our noses. You hold two mirrors 1 foot in front of your face. One is a plane mirror, and the other is a concave mirror with a 3-inch focal length. In which case are you more likely to have to strain your eyes to see the image of your nose?

4. When you mix red and green light from separate projectors, you get yellow spot on the wall. However, if you mix red and green paint, you get a muddy brown color. How do you account for this difference?