1969-02-26 — Page 14

F=-700 ETTAIEDZ71 WAH KIU YAT PO

1969

文中學會考試題預習

物理科

(十七) 陸永嫩

PHYSICS (17)

The laws of refraction are stated as follows: faw I. The incident and refracted rays are or

opposite sides of the normal at the point of incidence and all three are on the same plane,

11 The ratio of the sine of the engle of 1 incidence to the sine of the angle of refraction is a constant to a given pair of media. This law is also known as Shell's law.

Iractive: ANGA

The change in direction, of waves on coriaction depends on a change in speed. Since the amount of refraction depends on the amount of the change in speed, the ratio of speeds i ased as a measure of the degree of deflection. The index of refraction of light passing from one medium to another is defined as the ratio of the speeds of light in the two media

symbols

where Bu the refractiva, známk

Care the speade of In the corresponding media.

lince speed of wave freq, or wave

x wavelength of wave

郭日僑茶

the circle at C. 00 is the required retracted

ray and the refracted angle is 29°,

Taking 4/3 as the refractive index of Wa Lor

w, the real depth of the fish is

18x

The actual depth of the fish is 24 inches below water surface.

Since wa

the height of the

allw

fly above the water is

la: inches

The rly 19

inches above the water level

refractive ridex,

However, it is not necessary to know th speeds or wavelengths of light in two media order to calculate the index of refraction. Its value may be determine i

by geometrical optica m

as shown in figure.

When light is refractad

from me medium into another, the ratio of

the semi-chord, y, in

the first medium to the

seni-chord, sy in the

second andium is 17

gonstant. This rati

does not change with different angles of incidence.

It my also be defined as the ratio of th sine of the angle of incidence to the sine the angle of refraction for a ray passing. from one medium into another madium.

Determination of the refractive index of rilas

That is, the mirror appears to water level.

The apparent depth of the small

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Using a 45 right-angled przem as shown abova, a ray of light can be turn through

In the diagram, krun represents une pazn of the ray through the prizm, F. being the poin of total reflection. The normals at F and G. intersect at J. The maximum angle of emergence will be given by the minimum angle of total internal reflection at F, 10. by the critical angle at F

Lat the critical angle pe cy

incident angle at G be 2, and angle of

emergence be e..

Since the angle formed by the two normals is 300 (The prism is 60°), in triangle JFG an *JG = 1200

180° (120°c) #6094¢

For the refraction at

sine

1.5 sin 1 1.5 sin(609

alt z

419484

1.5 sin (600

1.5 in 1812

10.468%

27954

The maximum angle of emergence

Refrac

index of air-glass :

index:

refractive index of water-giss

The object level

The apparent distance apart of the object and mirror 15 - 6 - 9 an

Therefore, the image or the object 1s 9 cm. behind the mirror,

the apparent depth or this Langa 18 2 x 9.

Lind

A glass block is placed vertically over

i straight line ruled on a sheet of paper. A. pin on a sliding cork adjacent to the block then moved up and down until there is no parallax between it and the image of the line Seen through the block. Measurement of the real and apparent depth are then taken

real depth apparent depth

wen Light passes from a medium to another optically denser medium, it will be refracted away from the normal, If the angle of incidence becoming greater and greater, a critical point reaches where the refracted ray bents at 90°, that is, it emerges along the mutual surfece o the two media. With an angle of incidence greater than this, all the light is internally reflected. This condition is known as total internal refreation and the angle of incidence at which light emerges at 90° is known as the critical angle

AIR

Lat US represent the surface of separation between air and water and A ar. incident ray making an angle of 40o with the normal NO. SAC

Mlong US mark off OD numerically equal Do one refractive index on a convenient soar of 4/3 units. At D erect a perpendicular to cut OA at B. With centre 0 and radius OB, drai

■ pircle. Along 031 mark off OE in 1 unit. Through & erect another perpendicular to cut

AIR CATER

AIR

VATER

LIR

沸

0.83

The critical angle for water được

0.83 T 5003

00361

Parallel rays, incidence from the dot 0 will

be reflected cotally when they hits the other" two sides. Two dots will be seen when viewed from above.

Questions for next week 1

33. (a) In Fig. (1), cops the sketch and draw: ;

(1) the two refracted rays corresponding to the parallel rays 40 and CD incident on the converging (convex lens; (ii) the incident ray)

which produces the refracted ray BF, Where is the principal focus.

Fig. (ii), copy the sketch of the diverging lans and the rays, and draw in: (i) the

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