Reflection of Light Class 9 ICSE — Physics Chapter 7

Reflection of light class 9 ICSE is the subject of this chapter from your Physics book. It explains how light returns into the same medium after striking a surface, and how mirrors use that return to form images. The chapter moves from plane mirrors and the laws of reflection to spherical mirrors, ray diagrams, and the mirror formula. Each section is explained below in the order your book presents it.

Download Reflection of Light Class 9 ICSE Physics Chapter 7 PDF

Download the Reflection of Light Class 9 ICSE Physics chapter PDF to revise offline or print for practice. It is the full chapter as printed in the book — 26 pages, 27 sections, 80 questions.

What is inside Chapter 7

26 pages · 27 sections · 2 worked examples · 80 exercise questions

• EXERCISE 7(A): 24 questions
• Multiple choice type :: 4 questions
• EXERCISE 7(C): 35 questions
• Multiple choice type :: 17 questions

7.1 REFLECTION OF LIGHT

Reflection is the return of light into the same medium after it strikes a surface. The part that returns is the reflected light; the rest is absorbed or transmitted, depending on the surface. We see non-luminous objects only because they reflect light that then reaches our eyes. A plane mirror is a glass plate with a polished front surface and a silvered back surface that does the reflecting.

Reflection is of two kinds. Regular reflection happens on smooth polished surfaces like a plane mirror, where a parallel beam stays parallel and goes in one fixed direction. Irregular (diffused) reflection happens on rough surfaces like a wall, where rays scatter in all directions. Diffused light is what lets us see ordinary objects from any position. In the Class 9 ICSE syllabus, this is the first chapter on light.

7.2 SOME TERMS RELATED WITH REFLECTION

This section fixes the vocabulary used in every ray diagram. The incident ray strikes the surface; the point where it strikes is the point of incidence; the ray that leaves is the reflected ray. The normal is the perpendicular drawn to the surface at the point of incidence.

• Angle of incidence i: the angle the incident ray makes with the normal.
• Angle of reflection r: the angle the reflected ray makes with the normal.
• Plane of incidence: the plane containing the incident ray and the normal.
• Plane of reflection: the plane containing the reflected ray and the normal.

In the book’s figure, surface MM_1 receives ray AO at point O and reflects it as OB, with ON as the normal.

7.3 LAWS OF REFLECTION

Every reflecting surface obeys two laws. Law 1: the angle of incidence equals the angle of reflection, written \angle i = \angle r. Law 2: the incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

There is a special case worth remembering. A ray incident normally on a mirror has i = 0^\circ, so r = 0^\circ. The ray reflects back along its own path and retraces it exactly.

7.4 EXPERIMENTAL VERIFICATION OF THE LAWS OF REFLECTION

An experiment verifies both laws. A white sheet on a drawing board carries a line MM_1 with a point O and a line OA drawn at an angle below 90^\circ. A plane mirror stands on MM_1, and two pins P and Q are fixed on OA.

Two more pins R and S are placed in line with the images of P and Q seen in the mirror. Joining O to R and S gives the reflected ray OB.

The angles are measured and the experiment repeats for several values. For incident angles of 25^\circ, 35^\circ, 45^\circ and 55^\circ, the reflected angle matches every time, verifying law 1. Because all four pin tips lie on the plane of the paper, the rays and the normal share one plane, verifying law 2.

7.5 FORMATION OF IMAGE BY REFLECTION

Rays travel in all directions from each point of an illuminated object. To locate an image, you need at least two rays from the same object point. Where the two reflected rays meet — or appear to meet when produced backwards — is the image of that point. Joining the image points of all object points gives the full image.

7.6 IMAGE OF A POINT OBJECT FORMED BY A PLANE MIRROR

For a point object O in front of a plane mirror MM_1, the book takes two incident rays that reflect at points A and B. The reflected rays AC and BD obey the laws of reflection at each point. When produced backwards, they meet at a point I behind the mirror.

The point I is the image of O. It is virtual because the reflected rays never actually meet at I; they only appear to come from it when seen from between C and D.

7.7 IMAGE OF AN EXTENDED OBJECT FORMED BY A PLANE MIRROR

The point-object construction extends to a full object. For an extended object AB, the book constructs the images of the end points A and B separately, giving A' and B', then joins them. Every point between them forms an image point in the same way.

The key property is the equal-distance rule: the normal distance of each image point behind the mirror equals the normal distance of the matching object point in front, written BN = B'N.

7.8 POSITION OF IMAGE

This section proves that the image lies as far behind the mirror as the object is in front. A normally incident ray OF reflects back along FO. Another ray OA reflects along AC, obeying the law of reflection.

The proof uses alternate and corresponding angles to show \angle AOF = \angle AIF. Then triangles AOF and AIF are congruent because they share side FA and each has a right angle. Hence OF = IF, which is exactly the equal-distance result.

7.9 LATERAL INVERSION

Lateral inversion is the interchange of the left and right sides in a plane mirror image. A ring on your left hand appears on the right hand of your image, and a pocket on the left appears on the right. A letter P appears as q in the mirror.

Some letters keep their shape because they are symmetrical about a vertical line: A, H, I, M, O, T, U, V, W, X, Y. Ambulances carry the word AMBULANCE written laterally inverted on the front, so a driver ahead reads it correctly in the rear view mirror. Images formed by spherical mirrors are also laterally inverted.

7.10 CHARACTERISTICS OF THE IMAGE FORMED BY A PLANE MIRROR

A plane mirror image is erect, virtual, the same size as the object, and laterally inverted. It sits at the same perpendicular distance behind the mirror as the object sits in front.

There is a displacement rule. If the object shifts a distance d toward the mirror, the image shifts d as well, so the object-image separation falls by 2d. An object moving at speed v makes the image appear to move at speed 2v.

Worked pattern: a boy stands 3 m from a mirror. His image appears 6 m away from him. If he walks 1 m closer, his distance to the mirror becomes 2 m and the image appears 4 m from him.

7.11 IMAGES FORMED IN TWO INCLINED MIRRORS

An object between two inclined mirrors forms many images, because the image from one mirror acts as an object for the other. The count depends on the angle \theta between the mirrors through n = \frac{360^\circ}{\theta}.

• If n is odd: images = n for an asymmetric object, and n - 1 for a symmetric object on the bisector.
• If n is even: images = n - 1 for all positions.

Worked pattern: for \theta = 45^\circ, n = 8, which is even, so 7 images form for any object position. The object and all images lie on a circle centred at the mirrors’ intersection.

7.12 IMAGES FORMED IN A PAIR OF MIRRORS PLACED PARALLEL TO EACH OTHER

For parallel mirrors, \theta = 0^\circ, so n = \frac{360^\circ}{0^\circ} is infinite. An object between them forms an infinite number of images. Each image acts as a virtual object for the opposite mirror, generating the next image in the chain.

In practice only a few images are clearly seen. Every reflection absorbs some light, so brightness falls with each bounce. Barber shops and showrooms use this arrangement. A thick plane mirror also forms multiple images; the second image, from the back silvered surface, is the brightest.

7.13 IMAGES FORMED BY TWO MIRRORS PLACED PERPENDICULAR TO EACH OTHER

For \theta = 90^\circ, the formula gives n = \frac{360^\circ}{90^\circ} = 4, so the number of images is n - 1 = 3. The image from one mirror acts as a virtual object for the other, and two of the resulting images overlap at the same point.

Geometrically, the object and all three images lie on a circle whose centre is the intersection point of the mirrors and whose radius is the object’s distance from that point.

7.14 USES OF PLANE MIRROR

Plane mirrors appear throughout daily life. As a looking glass, they show a full upright image. An optician places a mirror on the front wall so a sign board behind the patient appears at nearly double the room’s length. A barber uses two facing mirrors to show the back of the head.

• Periscope: two parallel mirrors inclined at 45^\circ.
• Kaleidoscope: three mirrors at 60^\circ, giving hexagonal patterns.
• Solar devices: a mirror directs sunlight onto the substance to be heated.

7.15 SPHERICAL MIRRORS

A spherical mirror is a reflecting surface that is part of a sphere, made by silvering part of a hollow glass sphere. There are two types. A concave mirror is silvered on the outer bulging surface, so reflection happens on the inner surface. A convex mirror is silvered on the inner surface, so reflection happens on the bulging outer surface.

This is where the chapter turns from plane to curved surfaces. For more help on the surrounding material, see the Class 9 ICSE Physics chapter collection.

7.16 BRIEF INTRODUCTION OF TERMS RELATED TO A SPHERICAL MIRROR

Five terms describe any spherical mirror. The centre of curvature C is the centre of the sphere of which the mirror is a part; the normal at any point on the mirror passes through it. The radius of curvature R is the distance from C to any point on the mirror surface.

The pole P is the geometric centre of the spherical surface. The aperture is the plane surface area through which light enters. The principal axis is the line joining P to C.

7.17 REFLECTION OF LIGHT RAY FROM A SPHERICAL MIRROR

Reflection from a spherical mirror obeys the same two laws as a plane surface. The method to draw the reflected ray is to join the point of incidence to the centre of curvature C; that line is the normal. Then draw the reflected ray on the other side of the normal so that \angle i = \angle r.

7.18 FOCUS AND FOCAL LENGTH

Rays parallel to the principal axis behave differently on each mirror. In a concave mirror they converge after reflection to a real focus F in front of the mirror. In a convex mirror the reflected rays appear to come from a virtual focus F behind the mirror. The focal length f is the distance PF from pole to focus.

The focal plane is the plane through F normal to the principal axis. A parallel beam inclined to the axis is focused at a point in this plane.

7.19 CONVENIENT RAYS FOR THE CONSTRUCTION OF IMAGE BY RAY DIAGRAM

Any two of four convenient rays can construct an image. Each has a fixed behaviour after reflection.

• A ray through the centre of curvature reflects back along its own path.
• A ray parallel to the principal axis passes through the focus (concave) or appears to come from it (convex).
• A ray through the focus becomes parallel to the principal axis.
• A ray incident at the pole reflects with the principal axis as normal, so \angle i = \angle r.

7.20 RAY DIAGRAMS FOR FORMATION OF IMAGES IN A CONCAVE MIRROR

A concave mirror forms different images for different object positions. The book works through all seven cases in order. The table summarises the position, size, and nature of the image for each case.

The only virtual case in this ICSE chapter is the object between F and P; every other position gives a real, inverted image. Memory aid: as the object moves from infinity toward the pole, the image moves the opposite way — from the focus to infinity — and only after the object passes the focus does the image jump behind the mirror.

7.21 RAY DIAGRAM FOR FORMATION OF IMAGE IN A CONVEX MIRROR

A convex mirror behaves the same for every object position. One ray parallel to the principal axis appears to come from the focus after reflection; another ray directed at the centre of curvature reflects back along its own path. Produced backwards, the reflected rays meet at a virtual image between pole and focus.

So the rule is constant: the image is virtual, upright, diminished, and lies between pole and focus. As the object moves closer, the image shifts from the focus toward the pole and grows.

7.22 RELATIONSHIP BETWEEN THE FOCAL LENGTH AND RADIUS OF CURVATURE

For any spherical mirror, the focal length is half the radius of curvature:

f = \frac{1}{2} R

The proof follows from the law of reflection. A ray parallel to the principal axis reflects through F, and angle chasing shows triangle FDC is isosceles, so DF = FC. For a small aperture, D lies close to P, giving PF = FC and hence 2PF = PC.

Worked pattern: a concave mirror with R = 30 cm has f = 15 cm. A convex mirror with f = 12 cm has R = 24 cm.

7.23 SIGN CONVENTION FOR THE MEASUREMENT OF DISTANCES

The Cartesian sign convention makes the mirror formula work. Three rules govern it.

• All distances are measured from the pole taken as origin, with light incident from the left.
• Distances in the direction of the incident light are positive; those opposite are negative.
• Distances above the principal axis are positive; those below are negative.

This makes the focal length of a concave mirror negative and that of a convex mirror positive. The object is placed to the left of the mirror so the convention operates. Common mistakes: forgetting that u is always negative, and using a positive f for a concave mirror.

7.24 FORMULAE FOR THE SPHERICAL MIRROR

The mirror formula relates object distance u, image distance v, and focal length f:

\frac{1}{f} = \frac{1}{u} + \frac{1}{v}

For a concave mirror, u and f are negative; v is negative for a real image and positive for a virtual image. For a convex mirror, u is negative while v and f are positive.

Linear magnification is the ratio of image length to object length:

m = \frac{I}{O} = -\frac{v}{u}

Magnification is negative for a real, inverted image and positive for a virtual, erect image.

Worked numerical: an object is 30 cm from a concave mirror of focal length 10 cm. Here f = -10 cm and u = -30 cm. Then \frac{1}{v} = \frac{1}{-10} - \frac{1}{-30} = -\frac{1}{15}, so v = -15 cm. The image lies 15 cm in front of the mirror. Magnification m = -\frac{-15}{-30} = -\frac{1}{2}, so the image is real, inverted, and half the object’s size.

7.25 USES OF SPHERICAL MIRRORS

Concave mirrors concentrate light or magnify nearby objects. A shaving mirror is held so the face lies between pole and focus, giving an upright magnified image. In a torch or headlight, the bulb sits at the focus so the reflected rays form a parallel beam. A dentist’s head mirror focuses a parallel beam onto a small area.

Convex mirrors spread light or widen the view. A street lamp reflector diverges light over a large area. A rear view mirror gives a virtual, upright, diminished image with a wider field of view than a plane mirror of the same size.

7.26 DISTINCTION BETWEEN A PLANE MIRROR, CONCAVE MIRROR AND CONVEX MIRROR (WITHOUT TOUCHING)

Hold the mirror near your face and move it slightly away. The behaviour of the face image identifies the mirror.

• Plane mirror: upright image of the same size that does not change size.
• Concave mirror: upright image that enlarges as the mirror moves away.
• Convex mirror: upright image that shrinks as the mirror moves away.

7.27 DIFFERENCE BETWEEN A CONCAVE AND CONVEX MIRROR

The two spherical mirrors differ in construction, reflection behaviour, and image formation.

What to practise from Reflection of Light Class 9 ICSE

Reflection of light Class 9 ICSE carries four exercise blocks. Work through them in order after reading the sections above.

• EXERCISE 7(A) — 24 questions on plane mirror reflection, laws, and image formation.
• Multiple choice type — 4 questions on laws and plane mirror images.
• EXERCISE 7(C) — 35 questions on spherical mirrors, ray diagrams, and the mirror formula.
• Multiple choice type — 17 questions on spherical mirror behaviour and numericals.

Most numerical questions draw on sections 7.22 and 7.24, while ray-diagram drawing questions come from sections 7.20 and 7.21. Confirm the chapter requirements on the CISCE board website. For the surrounding material, see the next chapter on the propagation of sound waves.

Reflection of Light Class 9 ICSE frequently asked questions

What are the two laws of reflection of light class 9 icse?

The first law states that the angle of incidence equals the angle of reflection, \angle i = \angle r. The second law states that the incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

How many images are formed when two plane mirrors are placed at an angle of 60 degrees?

Here n = \frac{360^\circ}{60^\circ} = 6, which is even, so the number of images is always n - 1 = 5. Five images form for any position of the object between the mirrors.

What is the difference between a real image and a virtual image in reflection of light class 9 icse?

A real image forms where reflected rays actually meet, can be caught on a screen, and is inverted. A virtual image forms where reflected rays only appear to meet when produced backwards, cannot be caught on a screen, and is erect.

When does a concave mirror form a virtual image in class 9 icse physics?

A concave mirror forms a virtual, upright, magnified image only when the object lies between the focus and the pole. For every object position at or beyond the focus, the image is real and inverted.

What is the mirror formula and magnification used in class 9 icse physics?

The mirror formula is \frac{1}{f} = \frac{1}{u} + \frac{1}{v}, where u is the object distance, v is the image distance, and f is the focal length. Linear magnification is m = \frac{I}{O} = -\frac{v}{u}, negative for a real image and positive for a virtual image.

Reference: ICSE Class 9 Physics textbook material, chapter 7: Reflection of Light — CISCE-aligned.

More from this book

• Chapter 1: Measurements and Experimentation
• Chapter 2: Motion in One Dimension
• Chapter 3: Laws of Motion
• Chapter 4: Pressure in Fluids and Atmospheric Pressure
• Chapter 5: Upthrust in Fluids Archimedes Principle and Floatation
• Chapter 6: Heat and Energy
• Chapter 8: Propagation of Sound Waves
• Chapter 9: Current Electricity
• Chapter 10: Magnetism