Reflection - light "bouncing" off a reflective surface. This obeys a simple law, the law of reflection!
The incident (incoming) angle equals the reflected angle. Angles are generally measured with respect to a "normal" line (line perpendicular to the surface).
Note that this works for curved mirrors as well, though we must think of a the surface as a series of flat surfaces - in this way, we can see that the light can reflect in a different direction, depending on where it hits the surface of the curved mirror.
So - light reflects from mirrors, according to the law of reflection. However, if the mirrors is curved, light still obeys this rule - it just looks a bit different. You have to visualize the curved mirror as a series of little flat mirrors.
So - light reflects from mirrors, according to the law of reflection. However, if the mirrors is curved, light still obeys this rule - it just looks a bit different. You have to visualize the curved mirror as a series of little flat mirrors.
REFRACTION - light changing mediums, such as going from air into glass (or water, plastic, etc.)
Consider a wave hitting a new medium - one in which is travels more slowly. This would be like light going from air into water. The light has a certain frequency (which is unchangeable, since its set by whatever atomic process causes it to be emitted). The wavelength has a certain amount set by the equation, c = f l, where l is the wavelength (Greek symbol, lambda).
When the wave enters the new medium it is slowed - the speed becomes lower, but the frequency is fixed. Therefore, the wavelength becomes smaller (in a more dense medium).
Note also that the wave becomes "bent." Look at the image above: in order for the wave front to stay together, part of the wave front is slowed before the remaining part of it hits the surface. This necessarily results in a bend.
MORE DETAIL, for the mathematically inclined:
The general rule - if a wave is going from a lower density medium to one of higher density, the wave is refracted TOWARD the normal (perpendicular to surface) line. See picture above.
Refraction is much different than reflection. In refraction, light enters a NEW medium. In the new medium, the speed changes. We define the extent to which this new medium changes the speed by a simple ratio, the index of refraction:
n = c/v
In this equation, n is the index of refraction (a number always 1 or greater), c is the speed of light (in a vacuum) and v is the speed of light in the new medium.
The index of refraction for some familiar substances:
vacuum, defined as 1
air, approximately 1
water, 1.33
glass, 1.5
polycarbonate ("high index" lenses), 1.67
diamond, 2.2
The index of refraction is a way of expressing how optically dense a medium is. The actual index of refraction (other than in a vacuum) depends on the incoming wavelength. Different wavelengths have slightly different speeds in (non-vacuum) mediums. For example, red slows down by a certain amount, but violet slows down by a slightly lower amount - meaning that red light goes through a material (glass, for example) a bit faster than violet light. Red light exits first.
In addition, different wavelengths of light are "bent" by slightly different amounts. This is trickier to see, but it causes rainbows and prismatic effects.
Some animation, etc.:
http://faraday.physics.utoronto.ca/PVB/Harrison/Flash/Waves/Refraction/Refraction.html
http://www.animations.physics.unsw.edu.au/jw/light/Snells_law_and_refraction.htm
http://www.freezeray.com/flashFiles/Refraction2.htm
And all of this helps explain how lenses form images.
As shown and discussed in class, light refracts TOWARD a normal line (dotted line on the left image, perpendicular to surface of lens) when entering a more dense medium.
Note in this convex lens that this direction of bend changes from down (with the top ray) to up with the bottom ray. This is due to the geometry of the lens. Look at the picture to make sure that this makes sense. As a result, the rays will intersect after leaving the lens. An image can form!
The FOCAL LENGTH (f) of a lens (or curved mirror) where the light rays would intersect, but ONLY IF THEY WERE INITIALLY PARALLEL to each other. Otherwise, they intersect at some other point, or maybe not at all (if the object is too close to be focused on)!
Note that your (human) eye lenses are convex - slightly thicker in the middle. Thus, your eyes form "real" images on the retina - upside-down! Unless, of course, the object is too close.
If an image is projected onto a screen, the image is REAL. Convex lenses (fatter in the middle) CAN create real images - the only cases where there are no images for convex lenses are when the object distance (between object and lens) is equal to the f, or when do < f. In the first case, there is NO image at all. In the second case, there is a magnified upright virtual image "inside" the lens.
Concave lenses (thinner in the middle) NEVER create real images and ONLY/ALWAYS create virtual images.
Top image depicts parallel light rays hitting a convex lens and meeting at the "focal point." A real image forms at the focal length of a convex lens, WHEN THE RAYS ARE INITIALLY PARALLEL. People who are farsighted wear convex lenses.
The bottom image depicts parallel light rays hitting a concave lens and diverging. In this case, under all circumstances (regardless of where the object is), only virtual images are formed. These can not be projected onto a screen - rather, they appear to reside "inside" the lens. People who are nearsighted wear concave lenses.
Consider a wave hitting a new medium - one in which is travels more slowly. This would be like light going from air into water. The light has a certain frequency (which is unchangeable, since its set by whatever atomic process causes it to be emitted). The wavelength has a certain amount set by the equation, c = f l, where l is the wavelength (Greek symbol, lambda).
When the wave enters the new medium it is slowed - the speed becomes lower, but the frequency is fixed. Therefore, the wavelength becomes smaller (in a more dense medium).
Note also that the wave becomes "bent." Look at the image above: in order for the wave front to stay together, part of the wave front is slowed before the remaining part of it hits the surface. This necessarily results in a bend.
MORE DETAIL, for the mathematically inclined:
The general rule - if a wave is going from a lower density medium to one of higher density, the wave is refracted TOWARD the normal (perpendicular to surface) line. See picture above.
Refraction is much different than reflection. In refraction, light enters a NEW medium. In the new medium, the speed changes. We define the extent to which this new medium changes the speed by a simple ratio, the index of refraction:
n = c/v
In this equation, n is the index of refraction (a number always 1 or greater), c is the speed of light (in a vacuum) and v is the speed of light in the new medium.
The index of refraction for some familiar substances:
vacuum, defined as 1
air, approximately 1
water, 1.33
glass, 1.5
polycarbonate ("high index" lenses), 1.67
diamond, 2.2
The index of refraction is a way of expressing how optically dense a medium is. The actual index of refraction (other than in a vacuum) depends on the incoming wavelength. Different wavelengths have slightly different speeds in (non-vacuum) mediums. For example, red slows down by a certain amount, but violet slows down by a slightly lower amount - meaning that red light goes through a material (glass, for example) a bit faster than violet light. Red light exits first.
In addition, different wavelengths of light are "bent" by slightly different amounts. This is trickier to see, but it causes rainbows and prismatic effects.
Some animation, etc.:
http://faraday.physics.utoronto.ca/PVB/Harrison/Flash/Waves/Refraction/Refraction.html
http://www.animations.physics.unsw.edu.au/jw/light/Snells_law_and_refraction.htm
http://www.freezeray.com/flashFiles/Refraction2.htm
And all of this helps explain how lenses form images.
Some animation, etc.:
http://faraday.physics.utoronto.ca/PVB/Harrison/Flash/Waves/Refraction/Refraction.html
http://www.animations.physics.unsw.edu.au/jw/light/Snells_law_and_refraction.htm
http://www.freezeray.com/flashFiles/Refraction2.htm
And all of this helps explain how lenses form images.
As shown and discussed in class, light refracts TOWARD a normal line (dotted line on the left image, perpendicular to surface of lens) when entering a more dense medium.
Note in this convex lens that this direction of bend changes from down (with the top ray) to up with the bottom ray. This is due to the geometry of the lens. Look at the picture to make sure that this makes sense. As a result, the rays will intersect after leaving the lens. An image can form!
The FOCAL LENGTH (f) of a lens (or curved mirror) where the light rays would intersect, but ONLY IF THEY WERE INITIALLY PARALLEL to each other. Otherwise, they intersect at some other point, or maybe not at all (if the object is too close to be focused on)!
Note that your (human) eye lenses are convex - slightly thicker in the middle. Thus, your eyes form "real" images on the retina - upside-down! Unless, of course, the object is too close.
If an image is projected onto a screen, the image is REAL. Convex lenses (fatter in the middle) CAN create real images - the only cases where there are no images for convex lenses are when the object distance (between object and lens) is equal to the f, or when do < f. In the first case, there is NO image at all. In the second case, there is a magnified upright virtual image "inside" the lens.
Concave lenses (thinner in the middle) NEVER create real images and ONLY/ALWAYS create virtual images.
Top image depicts parallel light rays hitting a convex lens and meeting at the "focal point." A real image forms at the focal length of a convex lens, WHEN THE RAYS ARE INITIALLY PARALLEL. People who are farsighted wear convex lenses.
The bottom image depicts parallel light rays hitting a concave lens and diverging. In this case, under all circumstances (regardless of where the object is), only virtual images are formed. These can not be projected onto a screen - rather, they appear to reside "inside" the lens. People who are nearsighted wear concave lenses.
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