# HC Verma solutions for Class 11, Class 12 Concepts of Physics Vol. 1 chapter 22 - Photometry [Latest edition]

## Chapter 22: Photometry

Short Answers [Page 453]

### HC Verma solutions for Class 11, Class 12 Concepts of Physics Vol. 1 Chapter 22 Photometry Short Answers [Page 453]

Short Answers | Q 1 | Page 453

What is the luminous flux of a source emitting radio waves?

Short Answers | Q 2 | Page 453

The luminous flux of a 1 W sodium vapour lamp is more than that of a 10 kW source of ultraviolet radiation. Comment.

Short Answers | Q 3 | Page 453

Light is incident normally on a small plane surface. If the surface is rotated by an angle of 30° about the incident light, does the illuminance of the surface increase, decreases or remain same? Does your answer change if the light did not fall normally on the surface?

Short Answers | Q 4 | Page 453

A bulb is hanging over a table. At which portion of the table is the illuminance maximum? If a plane mirror is placed above the bulb facing the table, will the illuminance on the table increase?

Short Answers | Q 5 | Page 453

The sun is less bright at morning and evening as compared to at noon although its distance from the observer is almost the same Why?

Short Answers | Q 6 | Page 453

Why is the luminous efficiency small for a filament bulb as compared to a mercury vapour lamp?

Short Answers | Q 7 | Page 453

The yellow colour has a greater luminous efficiency as compared to the other colours. Can we increase the illuminating power of a white light source by putting a yellow plastic paper around this source?

MCQ [Page 454]

### HC Verma solutions for Class 11, Class 12 Concepts of Physics Vol. 1 Chapter 22 Photometry MCQ [Page 454]

MCQ | Q 1 | Page 454

The one parameter that determines the brightness of a light source sensed by an eye is ____________ .

• energy of light entering the eye per second

• wavelength of the light

• total radiant flux entering the eye

• total luminous flux entering the eye

MCQ | Q 2 | Page 454

Three light sources A, B and C emit equal amount of radiant energy per unit time. The wavelengths emitted by the three source are 450 nm, 555 nm and 700 nm respectively. The brightness sensed by an eye for the sources are XA, XB and XC respectively. Then, ________ .

• XA > XB, XC > XB

• XA > XB, XB > XC

• XB > XA, XB > XC

• XB > XA, XC > XB

MCQ | Q 3 | Page 454

As the wavelength is increased from violet to red, the luminosity ____________ .

• continuously increases

• continuously decreases

• increases then decreases

• decreases then increases

MCQ | Q 4 | Page 454

An electric bulb is hanging over a table at a height of 1 m above it. The illuminance on the table directly below the bulb is 40 lux. The illuminance at a point on the table 1 m away from the first point will be about ___________ .

• 10 lux

• 14 lux

• 20 lux

• 28 lux

MCQ | Q 5 | Page 454

Light from a point source falls on a screen. If the separation between the source and the screen is increased by 1%, the illuminance will decrease (nearly) by ____________ .

• 0.5%

• 1%

• 2%

• 4%

MCQ | Q 6 | Page 454

A battery-operated torch is adjusted to send an almost parallel beam of light. It produces an illuminancle of 40 lux when the light falls on a wall 2 m away. The illuminance produced when it falls on a wall 4 m away is close to _________ .

• 40 lux

• 20 lux

• 10 lux

• 5 lux

MCQ | Q 7 | Page 454

The intensity produced by a long cylindrical light source at a small distance r from the source is proportional to _________ .

• $\frac{1}{r^2}$

• $\frac{1}{r^3}$

• $\frac{1}{r}$

• None of these

MCQ | Q 8 | Page 454

A photographic plate placed a distance of 5 cm from a weak point source is exposed for 3 s. If the plate is kept at a distance of 10 cm from the source, the time needed for the same exposure is _____________ .

• 3s

• 12s

• 24s

• 48s

MCQ | Q 9 | Page 454

A photographic plate is placed directly in front of a small diffused source in the shape of a circular disc. It takes 12s to get a good exposure. If the source is rotated by 60° about one of its diameter, the time needed to get the same exposure will be ___________ .

• 6s

• 12s

• 24s

• 48s

MCQ | Q 10 | Page 454

A point source of light moves in a straight line parallel to a plane table. Consider a small portion of the table directly below the line of movement of the source. The illuminance at this portion varies with its distance r from the source as ___________ .

• $1 \propto \frac{1}{r}$

• $1 \propto \frac{1}{r^2}$

• $1 \propto \frac{1}{r^3}$

• $1 \propto \frac{1}{r^4}$

MCQ | Q 11 | Page 454

Figure shows a glowing mercury tube. The intensities at point A, B and C are related as __________ .

• B > C > A

• A > C > B

• B = C > A

• B = C < A

MCQ [Page 454]

### HC Verma solutions for Class 11, Class 12 Concepts of Physics Vol. 1 Chapter 22 Photometry MCQ [Page 454]

MCQ | Q 1 | Page 454

The brightness-producing capacity of a source

(a) does not depend on its power

(b) does not depend on the wavelength emitted

(c) depends on its power

(d) depends on the wavelength emitted

MCQ | Q 2 | Page 454

A room is illuminated by an extended source. The illuminance at a particular portion of a wall can be increased by

(a) moving the source

(b) rotating the source

(c) bringing some mirrors in proper positions

(d) changing the colour of the source.

MCQ | Q 3 | Page 454

Mark the correct options.

(a) The luminous efficiency of a monochromatic source is always greater than that of a white light source of same power.

(b) The luminous efficiency of a monochromatic source of wavelength 555 nm is always greater than that of a white light source of same power.

(c) The illuminating power of a monochromatic source of wavelength 555 nm is always greater than that of a white light source of same power.

(d) The illuminating power of a monochromatic source is always greater than that of a white light source of same power.

MCQ | Q 4 | Page 454

Mark out the correct options.

(a) Luminous flux and radiant flux have same dimensions.

(b) Luminous flux and luminous intensity have same dimensions.

(c) Radiant flux and power have same dimensions.

(d) Relative luminosity is a dimensionless quantity.

[Page 455]

### HC Verma solutions for Class 11, Class 12 Concepts of Physics Vol. 1 Chapter 22 Photometry [Page 455]

Q 1 | Page 455

A source emits 45 joules of energy in 15 s. What is the radiant flux of the source?

Q 2 | Page 455

A photographic plate records sufficiently intense lines when it is exposed for 12 s to a source of 10 W. How long should it be exposed to a 12 W source radiating the light of same colour to get equally intense lines?

Q 3 | Page 455

Using figure, find the relative luminosity of wavelength (a) 480 nm, (b) 520 nm (c) 580 nm and (d) 600 nm.

Q 4 | Page 455

The relative luminosity of wavelength 600 nm is 0.6. Find the radiant flux of 600 nm needed to produce the same brightness sensation as produced by 120 W of radiant flux at 555 nm.

Q 5 | Page 455

The luminous flux of a monochromatic source of 1 W is 450 lumen watt−1. Find the relative luminosity at the wavelength emitted.

Q 6 | Page 455

A source emits light of wavelengths 555 nm and 600 nm. The radiant flux of the 555 nm part is 40 W and of the 600 nm part is 30 W. The relative luminosity at 600 nm is 0.6. Find (a) the total radiant flux, (b) the total luminous flux, (c) the luminous efficiency.

Q 7 | Page 455

A light source emits monochromatic light of  555 nwavelengthm. The source consumes 100 W of electric power and emits 35 W of radiant flux. Calculate the overall luminous efficiency.

Q 8 | Page 455

A source emits 31.4 W of radiant flux distributed uniformly in all directions. The luminous efficiency is 60 lumen watt−1. What is the luminous intensity of the source?

Q 9 | Page 455

A point source emitting 628 lumen of luminous flux uniformly in all directions is placed at the origin. Calculate the illuminance on a small area placed at (1.0 m, 0, 0) in such a way that the normal to the area makes an angle of 37° with the X-axis.

Q 10 | Page 455

The illuminance of  a small area changes from 900 lumen m−2 to 400 lumen m−2 when it is shifted along its normal by 10 cm. Assuming that it is illuminated by a point source placed on the normal, find the distance between the source and the area in the original position.

Q 11 | Page 455

A point source emitting light uniformly in all directions is placed 60 cm above a table-top. The illuminance at a point on the table-top, directly below the source, is 15 lux. Find the illuminance at a point on the table-top 80 cm away from the first point.

Q 12 | Page 455

Light from a point source falls on a small area placed perpendicular to the incident light. If the area is rotated about the incident light by an angle of 60°, by what fraction will the illuminance change?

Q 13 | Page 455

A student is studying a book placed near the edge of a circular table of radius R. A point source of light is suspended directly above the centre of the table. What should be the height of the source above the table so as to produce maximum illuminance at the position of the book?

Q 14 | Page 455

Figure shows a small diffused plane source S placed over a horizontal table-top at a distance of 2.4 m with its plane parallel to the table-top. The illuminance at the point Adirectly below the source is 25 lux. Find the illuminance at a point B of the table at a distance of 1.8 m from A.

Q 15 | Page 455

An electric lamp and a candle produce equal illuminance at a photometer screen when they are placed at 80 cm and 20 cm from the screen respectively. The lamp is now covered with a thin paper which transmits 49% of the luminous flux. By what distance should the lamp be moved to balance the intensities at the screen again?

Q 16 | Page 455

Two light sources of intensities 8 cd and 12 cd are placed on the same side of a photometer screen at a distance of 40 cm from it. Where should a 80 cd source be placed to balance the illuminance?

## HC Verma solutions for Class 11, Class 12 Concepts of Physics Vol. 1 chapter 22 - Photometry

HC Verma solutions for Class 11, Class 12 Concepts of Physics Vol. 1 chapter 22 (Photometry) include all questions with solution and detail explanation. This will clear students doubts about any question and improve application skills while preparing for board exams. The detailed, step-by-step solutions will help you understand the concepts better and clear your confusions, if any. Shaalaa.com has the CBSE Class 11, Class 12 Concepts of Physics Vol. 1 solutions in a manner that help students grasp basic concepts better and faster.

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Concepts covered in Class 11, Class 12 Concepts of Physics Vol. 1 chapter 22 Photometry are Light Process and Photometry, Refraction, Total Internal Reflection, Ray Optics - Mirror Formula, Reflection of Light by Spherical Mirrors, Refraction Through a Prism, Refraction by a Lens, Snell’s Law, Concave Mirror, Rarer and Denser Medium, Lensmaker's Formula, Thin Lens Formula, Lenses, The Microscope, Some Natural Phenomena Due to Sunlight, Dispersion by a Prism, Combination of Thin Lenses in Contact, Telescope, The Eye, Power of a Lens, Magnification, Refraction at Spherical Surfaces.

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