Lenses. Refraction (p 308) Refraction occurs when a wave changes the direction in which it is moving...
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Transcript of Lenses. Refraction (p 308) Refraction occurs when a wave changes the direction in which it is moving...
Lenses
Refraction• (p 308) Refraction occurs when a wave
changes the direction in which it is moving• This is caused by a change in speed as the
wave passes from one medium to another
• Similar to reflection, the angles of incidence and the angles of refraction are measured with respect to the normal
• (p308) the index of refraction the greater the greater the change in direction:
• Diamond: n = 2.42; ice: n = 1.31; air n = 1.00
• ( p 310) Snell’s Law:
• Question: what happens to the frequency of the waves or light when it passes from one medium to another?
• Answer: The frequency done not change
• (p 310) when light passes from a low index of refraction to a higher index of refraction, light slows down & bends toward the normal
•(add in margin) bending toward normal: from smaller “n” to a larger “n”
• (p 310) when light passes from a high index of refraction to a lower index of refraction, light speeds up & bends away from the normal•(add in margin) bending away from normal: from larger “n” to a smaller “n”
• (add to margin p 310)Example 1:
H2O
Air
Glass
air
Example 2: Example 3: (H2O drop)
Example 4: Example 5:white light rainbow
A ray on the normaldoes not bend
• Where would you have to aim the spear to catch the fish?
•(p 312) A pool of water is deeper than it appears because the light waves are sharter in water
Total internal reflection
• (add to p 313) total internal reflection only occurs when light rays are traveling from larger to smaller “n”
• The angle of incidence which results in a refracted angle of 90o is called the critical angle
Critical Angle Calculation • (add diagram to p315 #3) What is the critical
angle for air/lucite (n =1.51) interface?
• (p 312)The highway can appear to be wet. The wet road is a mirage. What is seen is a reflection of the sky on the hot road (or sand)
• The index of refraction of hot air is less that the index of refraction of cool air. Total reflection can occur-you see a reflection of the sky
• the twinkling of stars or distant lights is due to refraction. The direction of the light changes as it passes through the air at different temperatures
• You see an image of the sun for a few minutes before it rises and after it sets because the earth’s atmosphere refracts the light from the sun
Diagram:
Fiber Optics• (p 313) Fiber optics make use of total
internal reflection
• Fiber optics are replacing electric ciruits in communication technology
• Physicians use fiber optics to look inside your body
Refraction from Lenses• Lenses are a practical application of refraction• (p 326) there are 2 types of lenses:concave convex
•(p326)A convex lens is a converging lens: it brings light rays together
(Diagram p 327)
•(p 326) A concave lens is a diverging lens: it spreads out light rays
(Diagram p 327)
• (p 328) Because light can travel in either direction through a lens, we usually indicate a focal point on both sides of a lens
• A convex lens has a real focal point• A concave lens has a virtual focal point
•(add to margin p328)
concave convex
mirror f + f -
lens f - f +
Ray Diagram for Convex (Converging) Lenses
We will use the same rays as with mirrors
(p329a) Ray 1: parallel and through “f”
(p 330c) Ray 3:through “f” and then parallel
• (add p330)(remember rays pass through lenses but bounce off (reflect from) mirrors!)
• object outside “2f”: image inverted, smallerreal
• object at “2f’: image invertedsame sizereal
• Object between image inverted“2f” and “f” largerreal
• Object at “f” no image produced• Object inside “f” image erect
largervirtual
• The image is erect, larger and virtual
Ray Diagram for Convex Lens: object inside “f”
•We still follow the same pattern but we have to extend the refracted rays:
Ray Diagram for Diverging Lens
• (p 332) The image is erect, smaller, virtual
Chromatic Aberration (add to margin p333)
• Different wavelengths of light refracted by a lens focus at different points– Violet rays are refracted more
than red rays– The focal length for red light is
greater than the focal length for violet light
• Chromatic aberration can be minimized by the use of a combination of converging and diverging lenses
Sign Conventions for lenses• (p332) the characteristics of images formed
by convex lenses are the same as those formed by concave mirrors
• the characteristics of images formed by concave lenses are the same as those formed by convex mirrors
• The sign conventions for mirror and lenses are the same: real +
virtual –erect +inverted -
Lens Formulae
• Lens equation:
• Magnification:
• (p 333/334)See example problems 1 & 2