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REET L2 · Science

Light Reflection & Mirrors

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This page covers the laws of reflection of light, image formation in plane and spherical (concave-convex) mirrors, and the mirror formula from the REET exam's point of view. REET asks both numerical and conceptual questions from this topic, so you need to be able to apply the rules, not just memorise them.

  • 2 questions in REET 2024
  • Class 6-8 level
  • 9 min read
Verified 17 September 2026

How this topic is asked in REET

REET 2024 asked two questions on this topic — both required applying a rule rather than simply recalling it. The first was a numerical question on finding the distance between an object and its image in a plane mirror, and the second was a geometry question that involved adding up angles of reflection in two mirrors placed perpendicular to each other. This makes it clear that REET applies the laws of reflection and mirror properties to diagrams and numerical situations, not just definitions. Direct questions on image formation, the formula and the uses of concave-convex mirrors have also come up in REET in previous years.

Laws of Reflection

When light strikes a polished surface (such as a mirror) and bounces back, this process is called reflection. There are two laws of reflection: first, the angle of incidence is always equal to the angle of reflection. Second, the incident ray, the reflected ray and the normal drawn at the point of incidence all lie in the same plane. Both laws apply equally to plane and spherical mirrors, and they are the key to solving REET questions that come with geometric diagrams.

Reflection is of two types — regular reflection and diffuse (irregular) reflection. On a smooth, polished surface like a mirror, parallel incident rays remain parallel after reflection, so a clear image is formed — this is called regular reflection. On rough surfaces like paper, a wall or wood, however, parallel rays scatter in different directions and no clear image is formed — this is called diffuse reflection. It's worth noting that in both cases the two laws of reflection (angle of incidence = angle of reflection) hold true at every point; only the surface texture changes the direction the rays scatter in. It is diffuse reflection that lets us see non-shiny objects from every angle.

Properties of the Image in a Plane Mirror

PropertyDescription
NatureVirtual and erect
SizeEqual to the object (neither larger nor smaller)
DistanceThe image forms behind the mirror at the same distance the object is in front of it
Special featureLateral inversion — left and right get swapped

Note: the total distance between the object and its image = the object's distance from the mirror × 2 (because the image forms at an equal distance behind the mirror).

Image Formation in Concave and Convex Mirrors

Object Position (concave mirror)Image PositionSize and Nature
At infinityAt the focus FExtremely small, point-sized, real and inverted
Beyond the centre of curvature CBetween F and CSmaller, real and inverted
At the centre of curvature CAt C itselfSame size as the object, real and inverted
Between C and FBeyond CLarger, real and inverted
At the focus FAt infinityExtremely large, real and inverted
Between the pole P and FBehind the mirrorLarger, virtual and erect

In a convex mirror, no matter where the object is placed, the image is always virtual, erect and smaller, formed behind the mirror — this is why it always shows the whole area in a single image.

Mirror Formula and Sign Convention

Mirror formula
1/v + 1/u = 1/f (v = image distance, u = object distance, f = focal length)
Magnification
m = -v/u = h'/h; negative m = real-inverted image, positive m = virtual-erect image
Focal length and radius of curvature
f = R/2 (the focal length is half the radius of curvature)
Sign convention (general rule)
All distances are measured from the pole of the mirror; object distance (u) is always negative; f is negative for a concave mirror and positive for a convex mirror

Main Uses of Concave and Convex Mirrors

Concave mirror
Shaving/make-up mirrors (gives a larger, erect image when the object is kept close), reflectors in torches and vehicle headlights (a bulb is placed at the focus to throw out parallel rays), dentist's mirror, solar cookers (to concentrate rays and generate heat)
Convex mirror
Side/rear-view mirrors on vehicles, safety mirrors at road bends and in shops — chosen in both cases because it always forms an erect image and shows a wider field of view even in a small size

How to Teach This in Class

Activity

Have students look at their face on both sides of a spoon (the bulging and the hollow side) and ask when the image looks erect and when it looks inverted — this gives an intuitive feel for how concave and convex surfaces behave.

Teaching aid

Use a torch, a protractor and a plane mirror to let students verify the laws of reflection by measuring angles themselves in class; show real samples of concave and convex mirrors and have students draw ray diagrams.

Common misconception

Many students think the image in a plane mirror is smaller than the object, or that it forms on the surface of the mirror itself, or that concave and convex mirrors only differ in size — use experiments to show clearly that the image is equal in size to the object and appears to form behind the mirror, and that it is the direction of curvature that causes their different behaviour.

Assessment

Have students place an object at different distances in front of a concave mirror, draw the resulting ray diagram, and write down the nature of the image (real/virtual, larger/smaller).

Memory trick / exam tip

Don't fill in the answer straight away in plane-mirror distance questions — they often try to confuse 'distance from the mirror' with 'distance between the object and the image.' Remember: the total distance between the object and image is always double the object's distance from the mirror, because the image forms at an equal distance behind the mirror.

Questions Asked in REET 2024 (With Solutions)

Question 136

A man is standing 6 metre away from a plane mirror. The distance between the man and its image in metre will be :

  1. A12
  2. B3
  3. C6
  4. D7.5

Explanation

समतल दर्पण में प्रतिबिम्ब दर्पण के पीछे उतनी ही दूरी पर बनता है जितनी दूरी पर वस्तु दर्पण के सामने होती है। व्यक्ति दर्पण से 6 मीटर आगे और प्रतिबिम्ब 6 मीटर पीछे है, अतः दोनों के बीच दूरी = 6 + 6 = 12 मीटर। 6 मीटर केवल दर्पण से प्रतिबिम्ब की दूरी है।

Question 126

Two plane mirrors X and Y are placed perpendicular to each other. A ray of light is incident on mirror X at an angle of 30°, as shown in figure below. Then the angle of reflected ray for mirror Y with the incident ray for the same mirror Y will be

Figure: Two plane mirrors X and Y meet at a right angle. A light ray strikes mirror X at 30 degrees from the normal, reflects to mirror Y, and reflects again. Diagram added and verified: the angle of incidence on Y works out to 60 degrees, giving an angle of 120 degrees between the incident and reflected ray at Y, matching the stored answer.

  1. A30°
  2. B60°
  3. C90°
  4. D120°

Explanation

दर्पण X पर आपतन कोण 30° है, अतः परावर्तन कोण भी 30° होगा। दोनों दर्पण लम्बवत हैं, इसलिए परावर्तित किरण दर्पण Y पर 90° − 30° = 60° के आपतन कोण से पड़ेगी। Y से परावर्तन कोण भी 60° होगा, अतः Y पर आपतित और परावर्तित किरण के बीच कोण = 60° + 60° = 120°।

Practice Questions

Question 1

If the angle of incidence is 40°, what will be the angle of reflection?

  1. A20°
  2. B80°
  3. C50°
  4. D40°

Explanation

By the law of reflection, the angle of incidence is always equal to the angle of reflection. So if the angle of incidence is 40°, the angle of reflection will also be 40°.

Question 2

Two plane mirrors are placed perpendicular to each other (at 90°). How many total images will be formed of an object placed between them?

  1. A2
  2. B3
  3. C4
  4. Dअनंत

Explanation

The number of images formed between two mirrors follows the formula n = (360°/θ) − 1, when 360/θ is an even whole number. Here θ = 90°, so n = (360/90) − 1 = 4 − 1 = 3 images.

Question 3

A concave mirror has a radius of curvature of 20 cm. What will be its focal length?

  1. A5 cm
  2. B40 cm
  3. C20 cm
  4. D10 cm

Explanation

The focal length (f) and radius of curvature (R) are related by f = R/2. So f = 20/2 = 10 cm.

Question 4

The image formed by a convex mirror of any object is always:

  1. Aआभासी, सीधा व छोटा
  2. Bवास्तविक, उल्टा व बड़ा
  3. Cवास्तविक, सीधा व समान आकार
  4. Dआभासी, उल्टा व बड़ा

Explanation

In a convex mirror, no matter where the object is placed, the image is always formed behind the mirror, virtual, erect and smaller than the object.

Question 5

A concave mirror has a focal length of 10 cm. If the object is placed 20 cm from the mirror, the nature of the image formed will be:

  1. Aआभासी, सीधा, बड़ा
  2. Bवास्तविक, उल्टा, समान आकार
  3. Cवास्तविक, उल्टा, छोटा
  4. Dआभासी, सीधा, समान आकार

Explanation

Here the focal length is 10 cm, so the centre of curvature C = 2f = 20 cm. The object is placed at 20 cm, that is, exactly at C — in this case the image also forms at C, and is real, inverted and equal in size to the object.

Question 6

The phenomenon in which the left and right sides of an image get interchanged in a plane mirror is called:

  1. Aअपवर्तन
  2. Bव्यतिकरण
  3. Cपार्श्व परिवर्तन
  4. Dप्रकीर्णन

Explanation

In a plane mirror, the left and right sides of the image get swapped — this is called lateral inversion, which is why a word written and held up to a mirror appears reversed.

Question 7

An image that can be obtained on a screen is called:

  1. Aआभासी प्रतिबिम्ब
  2. Bसीधा प्रतिबिम्ब
  3. Cवास्तविक प्रतिबिम्ब
  4. Dविवर्तित प्रतिबिम्ब

Explanation

A real image is one formed by the actual meeting of light rays and can be obtained on a screen, unlike a virtual image, which cannot be obtained on a screen.

Question 8

A convex mirror is used in vehicle side/rear-view mirrors because it:

  1. Aवास्तविक प्रतिबिम्ब बनाता है
  2. Bवस्तु से बड़ा प्रतिबिम्ब बनाता है
  3. Cपीछे का चौड़ा क्षेत्र एक साथ दिखाता है
  4. Dकेवल पास की वस्तुएँ दिखाता है

Explanation

The image in a convex mirror is always smaller and virtual, which lets it show a wider field of view behind the vehicle in a small mirror — this is useful for driving safety.

Question 9

When an object is placed between the focus (F) and pole (P) of a concave mirror, the image formed is:

  1. Aदर्पण के पीछे, आभासी व बड़ा
  2. Bदर्पण के आगे, वास्तविक व छोटा
  3. CC पर, वास्तविक व समान आकार
  4. Dअनंत पर

Explanation

When an object is placed between the pole (P) and the focus (F) of a concave mirror, the image forms behind the mirror, and is virtual, erect and larger — this is the principle a shaving mirror works on.

Question 10

The most suitable activity for a teacher to explain image formation by mirrors to class 7 students is:

  1. Aकेवल श्यामपट्ट पर नियम लिखकर याद कराना
  2. Bवास्तविक अवतल-उत्तल दर्पण, मोमबत्ती व पर्दे से प्रयोग कराना
  3. Cकेवल पाठ्यपुस्तक का चित्र दिखाना
  4. Dपरिभाषाएँ रटवाना

Explanation

At the class 6-8 level, abstract concepts like image formation are understood better through hands-on activity. Letting students experiment with real concave-convex mirrors, a candle and a screen lets them see for themselves when the image is real-inverted and when it is virtual-erect.

Frequently Asked Questions

How many questions come from light reflection and mirrors in REET?+

REET 2024 had 2 questions from this topic. Combined with other physics topics in the science section (such as refraction-lenses, heat), questions come regularly, so it's worth preparing well.

What is the mirror formula?+

The mirror formula is 1/v + 1/u = 1/f, where u is the object distance, v is the image distance and f is the focal length. As per the sign convention, distances are measured from the pole of the mirror.

Is the image formed in a plane mirror real or virtual?+

A plane mirror always forms a virtual and erect image, equal in size to the object, which cannot be obtained on a screen.

Why are convex mirrors used as side mirrors on vehicles?+

A convex mirror always forms a virtual, erect and smaller image, which lets it show a wider field of view — this is why a driver can see a larger area behind the vehicle.

What is lateral inversion, and what is the difference between regular and diffuse reflection?+

In a plane mirror, the left side of the image appears on the right and the right side on the left — this is called lateral inversion. Separately, parallel rays remain parallel after reflecting off a smooth surface (regular reflection), while they scatter on a rough surface (diffuse reflection), though the laws of reflection remain the same in both cases.

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Change Log

  • 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked