L 33 Light and Optics [3] Law of reflection angle of incidence = angle of reflection • images formed by mirrors – plane mirrors R – curved mirrors PLANE • concave • convex • Images formed by lenses I • the human eye R – correcting vision problems • nearsightedness I I • farsightedness •astigmatism R – depth perception CONVEX CONCAVE
VIRTUAL image: light rays appear to come from a point where there is no light Image formation with lenses
REAL image: light rays actually converge at image location • converging lens • lenses are relatively PLANE MIRROR (positive lens) simple optical devices • Image is VIRTUAL • diverging lens • the principle behind • Same size as object • Upright (negative lens) the operation of a OBJECT IMAGE • Same distance behind lens is refractionÆ mirror • the human eye – correcting for the bending of light as it passes from air into CONCAVE MIRROR nearsightedness – correcting for glass (or plastic) • Image is REAL farsightedness • Upright • smaller than object OBJECT IMAGE • optical instruments
converging lens Diverging lens
F focal point F Æa converging lens focuses parallel rays to A diverging lens causes a point called the focal point. parallel rays to diverge as if they came from a Æ a thicker lens has a shorter focal length focal point F
1 Image formation by a A converging lens is used to converging lens focus rays from the sun to a point
image
object 2F F
ÎIf the object is located at a distance of at least 2F from the lens, the image is inverted and smaller than the object. since the sun is very ÎThe image is called a REAL image since light rays far from the lens, the actually converge at the image location rays are nearly parallel
converging lens is used in a camera to Image formation by a focus light onto the film diverging lens
Object image
when you focus a camera, ÎThe diverging lens produces an image that is upright you adjust the distance and diminished in size. between the lens and the ÎIt is a VIRTUAL image, since light rays do not film depending on the actually pass through the image point object location.
a magnifying lens Sight – the human eye
• Physics of the human eye • Abnormal vision F • Nearsightedness • Farsightedness F Object • astigmatism virtual image • Depth perception
By placing the lens close to the object we get a magnified virtual image.
2 • light enters through the The Eye the physics of the human eye cornea • the iris controls the amount of light that gets in, a muscle can close it or open it, the iris determines your eye color • the lens is filled with a jelly-like substance; the ciliary muscle can change the shape of the Æ by changing the focal The relaxed eye can easily focus on distant lens and thus change its length, (accommodation) the objects. To focus on close objects the lens is squeezed to focal length Lens is able to focus light onto shorten it’s focal length, making it possible to converge the The retina for objects located rays onto the retina. The near point is the distance at which at various distances the closest object can be seen clearly. It recedes with age.
When a nearsighted person views a distant object, the lens When a farsighted person tries to focus on a close object cannot relax enough to focus at the retina. The rays converge the lens cannot be squeezed enough to focus on the retina. too quickly. The remedy is to place a diverging lens in front The focusW point is behind the retina. The remedy is to place e of the eye to first diverge the rays before they enter the eye. a convergingh lens in front of the eye to converge the rays before they enter the eye.
How does the eye judge distance? astigmatism Right eye • Astigmatism means that the cornea is D B oval like a football instead of spherical like a basketball. Most astigmatic corneas Left eye have two curves – a steeper curve and a • Our brain interprets the images formed on the flatter curve. This causes light to focus on retinas of both eyes as a single image Æ this is more than one point in the eye, resulting in called binocular vision blurred vision. • Our eyes roll inward slightly to focus on the distant point D. Our brain interprets the distance BD by the muscular effort required to roll the eyes inward.
3 Astigmatism Down periscope
mirror
Object
Astigmatism can be corrected with specially Image shaped lenses or, in extreme cases, with surgery. mirror
Spherical or curved mirrors Where is the light bulb?
Concave mirror image of light bulb
light bulb F
Focus
f f parallel light rays are focused to one point A concave mirror will form a real image of an object placed at twice its focal length at a distance of twice the focal length. It will be inverted and the same size as the object.
convex mirror Dish antennas
signal from satellite
focus
detector at the focal parallel rays diverge from a focus behind point of the dish the mirror
4 Magnifying mirrors A concave mirror can provide a magnified image as used in this Homer’s cosmetic mirror. image Homer
when something placed within the focus of a concave mirror, an enlarged, upright image is formed. this principle is used in a shaving or makeup mirror
Convex mirrors: wide angle view
Object Image
A convex lens provides a wide angle view. Since it sees more, the images are reduced in size. Passenger side mirrors are often of this type with the warning: “objects appear further than they actually are". Because they appear smaller they look further away.
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