2. The time for photographic print is 10 sec at a distance of 2m from 40 cd lamp. The time required for exposing the same print at a distance of 4m from 20 cd lamp is
[BP 2010]
10 s
40 s
80 s
160 s
(c) Time ∝ (distance)2/intensity → (4/2)2×(40/20) = 4×2 = 8× original time = 80s
3. The luminous efficiency of a lamp is 50 lumen/watt and its power is 40 watt. Its luminous flux in lumen is
5. The illuminance at 10 m directly below a 40 cd lamp is
[MOE 2012]
4 lux
0.4 lux
40 lux
0.04 lux
(b) E = I/d2 = 40/102 = 0.4 lux
6. Luminous intensity of light source of illuminance 50 lux at distance of 2m is
[MOE 2011]
400
300
200
100
(c) I = E×d2 = 50×4 = 200 cd
7. Two bulbs A and B are placed respectively at 20 cm and 30 cm on opposite sides of an oily paper screen. The two sides of the screen are equally intense. The ratio of power of the bulb A to that of B will be
[MOE 2009]
4:9
9:4
2:3
3:2
(a) Power ratio = (distance ratio)2 = (20/30)2 = 4:9
8. The shutter of camera is opened for 20s at a distance of 2m from the lamp of illuminance of 20 cd. If the distance is made 4m from the lamp of illuminance 40 cd. What is the time of shutter opening?
[IOM 2007]
20 s
40 s
60 s
80 s
(a) Same illuminance (40/42 = 20/22 = 5 lux) → same exposure time
9. Illuminance at a point 2m from a source of light of luminous intensity 100 candela is
[IOM 1997]
50 lux
25 lux
50 Cd/m2
25 Cd/m2
(b) E = I/d2 = 100/4 = 25 lux
10. The time of exposure of camera is 4 sec. What will be the time of exposure if its aperture is doubled
[MOE 2008]
1 sec
2 sec
3 sec
8 sec
(a) Time ∝ 1/(aperture area) → 4× → 1 sec when diameter doubled
11. A photographer finds that for a certain aperture of his camera the correct exposure time is 0.25 sec. If the diameter of the aperture is doubled then the exposure time will be
[BPKIHS 2007]
0.125 sec
0.500 sec
1.00 sec
0.062 sec
(d) Time ∝ 1/(diameter)2 → 0.25/4 = 0.0625 sec
12. The surface area of an electric lamp is 40cm2. If its illuminance at a distance 1m is 2Lm/m2 then the luminous flux from the lamp is
[]
2π Lumen
4π Lumen
8π Lumen
16π Lumen
(c) Flux = E×4πd2 = 2×4π(1)2 = 8π lumen
13. The luminous flux from a 100 watt electric lamp is 6850 lumen. The luminous efficiency of the lamp is
15. What is the ratio of luminous intensity of two sources which produce shadows of equal intensities at distances of 50cm and 100cm from the photometer?
[]
1:2
1:4
4:1
2:1
(b) I ∝ d2 → ratio = (50/100)2 = 1:4
16. The maximum illumination on a screen at a distance of 2 meters from a lamp is 25 lux. The value of total luminous flux emitted by the lamp is
[]
1256 lumens
1600 lumens
100 candela
400 lumens
(a) Flux = E × 4πd2 = 25 × 4π × 4 ≈ 1256 lumens
17. The illuminance of a surface 2m away from a point source is 4W/m2. It will be 2W/m2 when the distance of the point from the source is:
[]
1 m
√2 m
2 m
2√2 m
(b) E ∝ 1/d2 → √2 times distance for half illuminance
18. The Illuminance at a point on a plane surface at a distance of 4m from a bulb is 10-4 lumen/cm2. The line joining the point to the bulb makes an angle of 60° with the normal. The luminous intensity of the bulb is
[]
80 Cd
40 Cd
160 Cd
320 Cd
(c) E = Icosθ/d2 → I = E·d2/cos60° = (10-4×104)×16/0.5 = 160 Cd
19. In a cinema hall the distance between the projector and the screen is increased by 2%. If other variables are kept fixed, then the intensity of the illumination on the screen
[]
decreases by 2%
decreases by 4%
increases by 2%
increases by 4%
(b) I ∝ 1/d2 → (1.02)-2 ≈ 1 - 0.04 (4% decrease)
20. A bulb is situated at a height of 2m above the centre of the table. If the height is decreased by 1m, then percentage change in illumination at the centre of the table will be
[]
100% decrease
300% increase
100% increase
300% decrease
(b) I ∝ 1/d2 → (2/1)2 = 4× → 300% increase
21. A 100 watt lamp has luminous intensity 125 candela (isotropic). The luminous flux of the lamp is
[]
400π lumen
125π lumen
5000π lumen
500π lumen
(d) Flux = 4πI = 4π × 125 = 500π lumen
22. At what distance 16 candela lamp should be placed from a book so that the illumination received is 1 lumen/m2?
[]
1 m
1/4 m
2 m
4 m
(d) d = √(I/E) = √(16/1) = 4 m
23. Two lamps A and B of 64 and 16 candela respectively are placed 4m apart. At what distance from A should the screen be placed between the lamps so that it is equally illuminated by both the lamps?
[]
8/3 m
4/3 m
4 m
8 m
(a) I1/d12 = I2/d22 → 64/x2 = 16/(4-x)2 → x = 8/3 m
24. A 100W lamp is suspended at a height of 5m above the centre of a table. The intensity at the centre of the table in watt/m2 is
25. A small bulb is hanging at a height of 8 feet above the centre of a round table of diameter 16 feet. The ratio of the intensity of illumination at the centre and at points on the circumference of the table will be:
[]
1:1
2:1
2√2:1
3:2
(c) Ecenter/Eedge = (8√2/8)3 = (2√2)/1 (for point sources)
26. The separation between the screen and perfectly reflecting plane mirror is 2r. An isotropic point source of light is placed exactly midway between the mirror and the screen. The ratio of Illuminance on the screen with and without the mirror is:
[]
10:1
2:1
10:9
9:1
(c) Without mirror: E = I/r2 With mirror: E = I/r2 + I/(3r)2 = 10I/9r2 Ratio = 10/9
27. A photographer finds that for a certain aperture of his camera the correct exposure time is 0.25sec. If the diameter of the aperture is doubled, then time of exposure will be
[]
0.125s
0.5s
1s
0.0625s
(d) Time ∝ 1/(diameter)2 → 0.25/4 = 0.0625 s
28. The correct exposure time for photographic print is 10sec at a distance of 2m from a 40 C.P. lamp. The time required for exposing the same print at a distance of 4m from 20C.P. lamp is:
