Law of reflection L 33 Light and Optics [3] 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 the 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 CONCAVE MIRROR nearsightedness it passes from air into – correcting for glass (or plastic) • Image is REAL • Upright farsightedness • smaller than object OBJECT IMAGE • optical instruments

Refraction (bending) depends upon converging lens the index of refraction, n

The larger the value of n, the more Refracted bending that occurs ray Normal line focal point F Æa converging lens focuses parallel rays to a point called the focal point. Incident Material of Index of refraction, n Æ a thicker lens has a shorter focal length ray

1 Diverging lens Image formation by a converging lens

image

F object 2F F

A diverging lens causes ÎIf the object is located at a distance of at least 2F from the parallel rays to diverge lens, the image is inverted and smaller than the object. ÎThe image is called a REAL image since light rays as if they came from a actually converge at the image location focal point F

A converging lens is used to converging lens is used in a camera to focus rays from the sun to a point focus light onto the film

when you focus a camera, since the sun is very you adjust the distance far from the lens, the between the lens and the rays are nearly film depending on the parallel object location.

Image formation by a a magnifying lens diverging lens

F

Object image F Object virtual image ÎThe diverging lens produces an image that is upright and diminished in size. By placing the lens close to the object ÎIt is a VIRTUAL image, since light rays do not actually pass through the image point we get a magnified virtual image.

2 Sight – the human eye • light enters through the The Eye cornea • the iris controls the • Physics of the human eye amount of light that gets in, a muscle can close it • Abnormal vision or open it, the iris • Nearsightedness determines your eye • Farsightedness color • astigmatism • the lens is filled with a jelly-like substance; the • Depth perception ciliary muscle can change the shape of the Æ by changing the focal lens and thus change its length, (accommodation) the focal length Lens is able to focus light onto The retina for objects located at various distances

the physics of the human eye

The relaxed eye can easily focus on distant objects. To focus on close objects the lens is squeezed to When a nearsighted person views a distant object, the lens shorten it’s focal length, making it possible to converge the cannot relax enough to focus at the retina. The rays converge rays onto the retina. The near point is the distance at which too quickly. The remedy is to place a diverging lens in front the closest object can be seen clearly. It recedes with age. of the eye to first diverge the rays before they enter the eye.

How does the eye judge distance? Right eye

D B

Left eye • Our brain interprets the images formed on the retinas of both eyes as a single image Æ this is called binocular vision When a farsighted person tries to focus on a close object • Our eyes roll inward slightly to focus on the the lens cannot be squeezed enough to focus on the retina. distant point D. Our brain interprets the distance The focusW point is behind the retina. The remedy is to place BD by the muscular effort required to roll the e eyes inward. a convergingh lens in front of the eye to converge the rays before they enter the eye.

3 astigmatism Astigmatism • Astigmatism means that the cornea is oval like a football instead of spherical like a basketball. Most astigmatic corneas have two curves – a steeper curve and a flatter curve. This causes light to focus on more than one point in the eye, resulting in blurred vision.

Astigmatism can be corrected with specially shaped lenses or, in extreme cases, with surgery.

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