ScienceCBSE Class 8 · Light: Mirrors and Lenses

Laws of Reflection and Angle Problems: Class 8 Science

Updated 17 October 2026 · Fact-checked

The two laws of reflection are: the angle of incidence equals the angle of reflection, and the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane. Both angles are measured from the normal, not from the mirror, and the laws hold for plane, concave and convex mirrors.

Notice that both i and r are measured from the normal, not from the mirror, and they are equal; all three lines lie in one plane.

Illustration drawn for QuestionBuddy.

Understand Laws of Reflection and Angle Problems

Rays and terms. Light travels in straight lines and is drawn as a line with an arrow, a ray. The ray falling on a mirror is the incident ray; the ray coming back is the reflected ray. The normal is the line drawn at 90 degrees to the mirror surface at the point where the incident ray strikes (point O). The angle of incidence (i) is the angle between the incident ray and the normal; the angle of reflection (r) is the angle between the reflected ray and the normal.

Activity 10.4. A thin beam from a torch passes through a comb with only one slit open, falls on an upright plane mirror, and is traced on white paper. Changing the direction of the incident beam shifts the reflected beam. Drawing the mirror line, the rays and the normal, and measuring i and r for several trials, the two angles come out nearly equal. If the beam falls along the normal, both angles are zero.

First law: the angle of incidence (i) is equal to the angle of reflection (r).

Activity 10.5. A reflected beam is seen on an extended part of the chart paper. When the extended part is bent along the table edge, the beam disappears; flattening the paper brings it back. So the reflected beam is in the same plane as the incident beam. Second law: the incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.

Validity. The laws hold for all kinds of mirrors, plane, concave and convex. In Activity 10.6 parallel beams reflected from a plane mirror stay parallel, but from a concave mirror they converge and from a convex mirror they diverge. Each ray still obeys the laws; it is the curved surface, whose normals point in different directions at different points, that makes the beams converge or diverge. The image in a plane mirror is erect, the same size as the object, and laterally inverted.

Working with angles.

Angle of reflection = angle of incidence.
Angle between the ray and the mirror surface = 90 degrees minus the angle from the normal.
Angle between the incident ray and the reflected ray = i + r = 2i.
A ray along the normal reflects straight back (i = r = 0 degrees).
For a ray falling at i = 40 degrees, the angle made by the reflected ray with the mirror is 90 − 40 = 50 degrees.

Image formation in curved mirrors and lenses is covered in the next topic.

Laws of Reflection and Angle Problems: formulas and key facts

First law of reflection: $A n g l eo f in c i d e n ce = A n g l eo f r e f l ec t i o n (i = r)$; Both angles measured from the normal.
Second law of reflection: Incident ray, normal and reflected ray lie in the same plane; Shown by bending the paper in Activity 10.5.
Angle with the mirror: $A n g l e w i t hmi r r or = 90 d e g r ees - an g l e f r o m t h e n or ma l$; Normal is perpendicular to the mirror.
Angle between incident and reflected rays: $i + r = 2 i$; Derived from the first law.
Normal incidence: $i = 0° \Rightarrow r = 0°$; The ray returns along itself.

How to solve Laws of Reflection and Angle Problems questions

Use this method for angle questions.

1Draw the mirror, mark the point O and draw the normal at 90 degrees.
2Locate the incident ray and mark i between it and the normal.
3If an angle is given with the mirror, find i = 90 degrees − that angle.
4Set r = i and mark the reflected ray on the other side of the normal.
5Find what is asked: r, angle with the mirror (90 − r), angle between the rays (2i), or the angle through which the ray turned.
6Write the law you used and give units (degrees).

Laws of Reflection and Angle Problems in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Torch beam on a mirror

Suppose a student shines a beam of torch light so that it makes an angle of 35 degrees with the surface of a plane mirror lying flat on a table.

(a) Find the angle of incidence. (b) Find the angle of reflection. (c) Find the angle between the incident and reflected rays. (d) What will the angle of reflection be if the beam is turned to fall along the normal?

Try it first, then show the solutionSolution

Angle from the normal: i = 90 − 35 = 55 degrees.
r = i = 55 degrees.
i + r = 55 + 55 = 110 degrees.
Along the normal, i = 0, so r = 0 degrees.

Answer: (a) 55 degrees; (b) 55 degrees; (c) 110 degrees; (d) 0 degrees.

Memory hook: Measure from the normal, never from the mirror.

Why the beam disappears when the paper bends

Imagine the chart paper in Activity 10.5 is bent upward at the table edge and the reflected beam is lost.

(a) State the law shown. (b) What happens when the paper is flattened again? (c) Is the law also true for a concave mirror?

Try it first, then show the solutionSolution

The incident ray, the normal and the reflected ray lie in the same plane.
The beam reappears on the paper because the plane is again the paper's plane.
Yes, the laws of reflection hold for plane, concave and convex mirrors.

Answer: (a) Second law of reflection; (b) beam reappears; (c) yes.

Memory hook: Three lines share one flat sheet.

Tricks to remember Laws of Reflection and Angle Problems

Short memory aids. Say each one aloud once, then use it on a question the same day.

Normal is the neighbour of the angles
Both i and r hug the normal.
Use it when: Where to measure from.
Ninety minus
If the question gives the angle with the mirror, subtract from 90 to get i.
Use it when: Angle conversions.
Two laws: equal angles, same plane
E and S: Equal and Same plane.
Use it when: Stating the laws.

Laws of Reflection and Angle Problems: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: State and illustrate the laws · 3 marks · Short answerBoard level

State the two laws of reflection and name the angle of incidence and angle of reflection with reference to the normal.

Try it first, then show the solutionSolution

Law 1: the angle of incidence is equal to the angle of reflection. (1)
Law 2: the incident ray, the normal at the point of incidence and the reflected ray lie in the same plane. (1)
The angle of incidence is between the incident ray and the normal; the angle of reflection is between the reflected ray and the normal. (1)

Answer: i = r; the three lie in one plane; both angles are from the normal.

Example 2: Angle of reflection · 1 mark · MCQBoard level

A ray makes an angle of 52 degrees with the normal at a mirror. The angle made by the reflected ray with the mirror is
(a) 52 degrees
(b) 104 degrees
(c) 38 degrees
(d) 76 degrees

Try it first, then show the solutionSolution

r = i = 52 degrees from the normal, so the angle with the mirror is 90 − 52 = 38 degrees.

Answer: (c)

Example 3: A sequence of angles · 5 marks · Long answerHarder board level

A ray strikes a plane mirror so that the angle between the incident ray and the reflected ray is 76 degrees.
(a) Find the angle of incidence. (1)
(b) Find the angle the incident ray makes with the mirror. (1)
(c) Find the angle the reflected ray makes with the mirror. (1)
(d) If the ray is turned so that the angle of incidence becomes 60 degrees, find the new angle between the incident and reflected rays. (1)
(e) Through what angle did the reflected ray turn between the two cases? (1)

Try it first, then show the solutionSolution

(a) i + r = 76 and i = r, so i = 38 degrees. (1)
(b) 90 − 38 = 52 degrees. (1)
(c) The same: 52 degrees (r = 38 degrees from the normal). (1)
(d) 2 × 60 = 120 degrees. (1)
(e) The reflected ray moved from 38 to 60 degrees from the normal on the other side, a change of 22 degrees. (1)

Answer: (a) 38 degrees; (b) 52 degrees; (c) 52 degrees; (d) 120 degrees; (e) 22 degrees.

Example 4: Reading the plane-of-reflection experiment · 4 marks · Case-basedHarder board level

Read the passage and answer.

In an experiment a beam of light falls on a mirror lying on a chart paper that extends beyond the table edge. The reflected beam is seen on the extended part. When the extended part is bent downward the beam is no longer seen on it.
(a) Name the law tested. (1)
(b) Explain the disappearance. (1)
(c) The incident beam makes 30 degrees with the normal. Give the angle of the reflected beam from the normal. (1)
(d) Where does the reflected ray lie if the incident beam is along the normal? (1)

Try it first, then show the solutionSolution

(a) The second law of reflection. (1)
(b) Bending the sheet makes a new plane, so the reflected ray no longer lies on it; flattening the sheet brings the beam back. (1)
(c) 30 degrees. (1)
(d) Along the same normal line, going back along the incident ray. (1)

Answer: (a) Second law; (b) bending changes the plane; (c) 30 degrees; (d) back along the normal.

Example 5: Parallel beams on different mirrors · 4 marks · Case-basedHOTS / competency

Read the passage and answer.

Three mirrors, plane, concave and convex, are each lit with several parallel beams. A student says: 'Spherical mirrors break the laws of reflection because the beams do not stay parallel.'
(a) State what happens to the reflected beams for each mirror. (1)
(b) Evaluate the student's statement. (1)
(c) A ray falls on a concave mirror at i = 25 degrees. Give r. (1)
(d) Why does the curved surface still produce convergence or divergence? (1)

Try it first, then show the solutionSolution

(a) Plane: remain parallel; concave: converge; convex: diverge. (1)
(b) Wrong: the laws of reflection are valid for all kinds of mirrors; each ray obeys them. (1)
(c) r = 25 degrees. (1)
(d) The curved surface has a different normal at each point, so the reflected rays leave in different directions and converge or diverge. (1)

Answer: (a) Parallel, converge, diverge; (b) laws still hold; (c) 25 degrees; (d) normals differ over the curved surface.

Where students lose marks in Laws of Reflection and Angle Problems

The slips examiners see most often, and what to write instead.

Mistake: Measuring the angle of incidence from the mirror surface.
The ray and the mirror seem the natural pair.
Wrong working
Ray makes 30 degrees with the mirror, so i = 30 degrees.
Right working
i is from the normal: i = 90 − 30 = 60 degrees.
Do this: Draw the normal first.
Usually costs: 1 to 2 marks.
Mistake: Saying the incident ray alone lies in the plane.
Second law is stated incompletely.
Wrong working
The reflected ray lies in the plane of the mirror.
Right working
The incident ray, the normal and the reflected ray all lie in the same plane.
Do this: Name all three.
Usually costs: 1 mark.
Mistake: Saying the laws fail for curved mirrors.
Parallel beams do not stay parallel.
Wrong working
Curved mirrors do not follow the laws of reflection.
Right working
The laws hold for all mirrors; the curved surface makes the normals differ, so beams converge or diverge.
Do this: Apply the laws to each ray.
Usually costs: 1 mark.
Mistake: Forgetting the normal in diagrams.
Only the mirror and rays are drawn.
Wrong working
A diagram with mirror, incident ray and reflected ray only.
Right working
A diagram with mirror, normal (dashed), incident ray with arrow, reflected ray with arrow, and i and r marked.
Do this: Always draw and label the normal.
Usually costs: 1 mark.

Exam tips

Draw and label the normal in every ray diagram.
State both laws completely.
Convert angles with the mirror to angles from the normal by subtracting from 90 degrees.
Add arrows to every ray.
Say that the laws are valid for plane and spherical mirrors.

Practice questions on Laws of Reflection and Angle Problems

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · Very short answerBoard level
What is the normal to a mirror at the point of incidence?
Hint
Perpendicular line.
Answer
A line drawn at 90 degrees to the mirror surface at the point where the incident ray strikes it.
Q2 · 1 mark · MCQBoard level
A ray of light falls along the normal on a plane mirror. The angle of reflection is
(a) 90 degrees
(b) 45 degrees
(c) 0 degrees
(d) 180 degrees
Hint
i = 0.
Answer
(c)
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The laws of reflection are valid for concave and convex mirrors.
Reason (R): Each ray of light obeys the laws of reflection at the point where it strikes the mirror.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Think of single rays.
Answer
(a) R explains A.
Q4 · 4 marks · Short answerHarder board level
A ray makes an angle of 28 degrees with the surface of a plane mirror. (a) Find the angle of incidence. (b) Find the angle of reflection. (c) Find the angle between the incident and reflected rays. (d) The mirror is replaced by one that is tilted so the angle of incidence becomes 50 degrees. Find the new angle between the incident and reflected rays.
Hint
90 − 28 = 62.
Answer
(a) 90 − 28 = 62 degrees. (b) 62 degrees. (c) 124 degrees. (d) 2 × 50 = 100 degrees (1 mark each).
Q5 · 4 marks · Case-basedHOTS / competency
Read the passage and answer.
A torch beam hits a plane mirror and the reflected beam is traced. The angle between the incident beam and the mirror surface is 70 degrees. The beam is then turned so that this angle reduces by 25 degrees.
(a) Find i before and after. (b) By how much did r change? (c) Find the angle between incident and reflected rays after the change. (d) Is the reflected ray still in the same plane as the incident ray and the normal?
Hint
Before: angle with mirror 70, after: 45.
Answer
(a) Before: 90 − 70 = 20 degrees. After: the angle with the mirror is 45 degrees, so i = 90 − 45 = 45 degrees. (b) r changed from 20 to 45 degrees, an increase of 25 degrees. (c) 2 × 45 = 90 degrees. (d) Yes, by the second law (1 mark each).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is the first law of reflection?Show answer
The angle of incidence equals the angle of reflection.
2.What is the second law?Show answer
The incident ray, the normal and the reflected ray lie in the same plane.
3.What is the angle of incidence for a ray along the normal?Show answer
0 degrees.
4.Do the laws apply to curved mirrors?Show answer
Yes, they are valid for plane, concave and convex mirrors.
5.How is light represented?Show answer
By straight lines with arrows, called rays.

Laws of Reflection and Angle Problems: frequently asked questions

Why is the normal drawn at 90 degrees?

It is defined as the line perpendicular to the reflecting surface at the point of incidence, and both angles are measured from it.

Is a plane mirror image laterally inverted?

Yes, a plane mirror image is laterally inverted (left and right appear interchanged).

Why does the beam in Activity 10.5 disappear?

Bending the paper moves it out of the plane that contains the incident ray, the normal and the reflected ray.

What is the use of the comb in the activity?

With one slit open it gives a thin beam; with many slits open it gives multiple parallel beams.