Secondary 3 Physics | Light: Reflection and Refraction — Tutor Lesson Prep Guide | Normal, refracted angle, apparent depth, total internal reflection

2026-11-11

Tutor lesson-prep guides · Secondary Physics

Students who label the angle of reflection between the ray and the mirror are not being careless; they have measured angles from the ground all their lives. This lesson does one thing only: it makes measuring from the normal automatic.

Objectives — By the end students can: (1) measure the angle of reflection from the normal and work out that a 35 degree incidence gives 110 degrees between the incident and reflected rays; (2) use n equals sin i over sin r to find that 40 degrees in air becomes 28.9 degrees in water with n equal to 1.33; (3) state that light travels at about 2.256 times 10 to the 8 metres per second in water, that a 1.5 metre pool looks about 1.13 metres deep, and that the critical angle from water to air is 48.8 degrees.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Measuring angles	What do the angles of a triangle add up to?	180 degrees, used to check a between-ray answer
Trigonometry keys	How do you press sin and inverse sin?	Check the calculator is in DEG mode first
Speed units	What does 3 times 10 to the 8 metres per second mean?	Light covers three hundred million metres each second

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Diagnose the habit	Ask students to draw one ray hitting a plane mirror and label the angle; collect the drawings without marking them	Draw the ray and label the reflected angle as they see it
5-13 min	One line: the normal	Draw the mirror and the normal, state that every angle is measured from the normal	Re-label the angle between the ray and the normal on their own diagram
13-28 min	Three scripts	Teach the law of reflection, refraction and apparent depth, total internal reflection; stop after each for recall	Say back that light slows in water so the refracted angle is smaller
28-36 min	Four question types	Model mirror distance, between-ray angle, refracted angle, apparent depth; one each	Do one of each and write the steps
36-42 min	Error contrast	Write the four common mistakes up and have students correct them	Correct each sentence and point at the normal
42-45 min	Exit check	Collect the three answers and sort the errors on the spot	Complete three questions alone

03 Teaching script (read it out)

Scene 1: Script 1: every angle starts at the normal, and reflection matches incidence

“The normal is the line perpendicular to the mirror, not the mirror itself. The angle of incidence sits between the incident ray and the normal; the angle of reflection sits between the reflected ray and the normal; they are always equal. At 35 degrees of incidence the ray makes 90 minus 35, so 55 degrees with the mirror, and 180 minus 70, so 110 degrees with the other ray. Labelling it wrong does not mean you cannot do optics; it means you measured from the wrong line.”

Is the angle of reflection measured from the mirror or from the normal?

Scene 2: Script 2: light slows in water, the refracted angle shrinks, the bottom looks shallower

“Light manages 3 times 10 to the 8 metres per second in a vacuum and only 2.256 times 10 to the 8 in water, because n equal to 1.33 divides the speed. Slower means it bends towards the normal, so the refracted angle is always smaller than the angle of incidence. The law is n equals sin i over sin r: at 40 degrees, sin 40 divided by 1.33 is about 0.483, and inverse sin gives 28.9 degrees. The same effect makes a pool look shallower: a real 1.5 metres appears as 1.5 divided by 1.33, about 1.13 metres.”

Does light speed up or slow down in water, and is the refracted angle bigger or smaller?

Scene 3: Script 3: past the critical angle nothing gets out

“Going from water to air, light moves from slow to fast and bends away from the normal. As the angle of incidence grows, the refracted angle reaches 90 degrees first, and beyond that everything is reflected back into the water. That is total internal reflection. The critical angle comes from sin c equals 1 over n: for water to air, sin c is 0.7519 and c is 48.8 degrees. Optical fibres and periscopes rely on it, so if a question asks why the surface goes dark at 50 degrees, the answer is that it is past the critical angle.”

Above what angle of incidence does light travelling from water to air only reflect?

04 Question types and worked steps

Type	Example	Steps	Answer
Plane mirror image	A student stands 1.2 metres in front of a plane mirror. How far is he from his image?	(1) The image sits equally far behind the mirror. (2) Image distance is 1.2 metres. (3) Add the two distances.	2.4 metres
Reflection and between-ray angle	The angle of incidence is 35 degrees. What is the angle between the incident and reflected rays?	(1) The angle of reflection is also 35. (2) Both are measured from the normal. (3) 180 minus 70.	110 degrees
Refracted angle	Light hits water (n equals 1.33) at 40 degrees. What is the refracted angle?	(1) Write n equals sin i over sin r. (2) sin 40 divided by 1.33. (3) Take inverse sin, one decimal place.	28.9 degrees
Apparent depth and critical angle	A pool is 1.5 metres deep. How deep does the bottom look, and what is the critical angle from water to air?	(1) Apparent depth is real depth over n. (2) 1.5 divided by 1.33. (3) sin c equals 1 over 1.33.	About 1.13 metres; critical angle 48.8 degrees

Sanity check (teach this): Sanity check: the refracted angle must be smaller than 40, so 56 means n was multiplied instead of divided. Apparent depth must be less than the real 1.5 metres. A critical angle must lie between 0 and 90 degrees. The 110 degree answer must exceed a single 35, so 70 means one ray was left out.

05 Independent practice (3 questions)

Question	Answer	What to watch
The angle of incidence is 25 degrees. What is the angle between the two rays?	130 degrees	Whether they compute 180 minus 2 times 25 rather than repeating 25
Light in water meets the surface at 60 degrees. What happens?	Total internal reflection; the light stays in the water	Whether they compare 60 with the 48.8 degree critical angle first
A pool is 1.5 metres deep. What is the apparent depth?	About 1.13 metres	Whether they divide by 1.33 instead of multiplying

06 Common mistakes and what to say

Mistake	Student writes	Say this
Measuring the reflected angle from the mirror	“The reflected angle is 55 degrees”	“55 is the angle with the mirror. Measure from the normal: 90 minus 55 is 35 degrees.”
Believing light speeds up in water	“Light is faster in water, so the refracted angle is bigger”	“n equals 1.33 means it is 1.33 times slower. Slower means bending towards the normal, so the refracted angle is smaller.”
Multiplying by n to get the refracted angle	“sin r equals 1.33 times sin 40”	“n is sin i over sin r, so sin r is sin i divided by n, never multiplied.”
Thinking total internal reflection always happens	“Light always reflects totally when it leaves water”	“Only past the 48.8 degree critical angle. Below that, light still refracts out.”

07 Homework (paper, section and question numbers)

Physics chapter index: the Light chapter, redo one of each question type from the lesson — Light chapter, questions 3, 5 and 8
Physics chapter index: the Waves chapter, preview the wave speed, frequency and wavelength relation — Waves chapter, questions 1 and 2

08 Exit check (last 5 minutes)

Question	Answer	Decision
The angle of incidence is 40 degrees. What is the angle of reflection?	40 degrees	Wrong: reteach measuring from the normal next lesson
Light goes from air into water. Is the refracted angle bigger or smaller than the angle of incidence?	Smaller	Wrong: reteach speed and n from script 2
What is the critical angle from water to air?	About 48.8 degrees	Wrong: reteach total internal reflection from script 3

09 Tutor FAQ

The class has not met trigonometry yet. Is this guide still usable?

Yes. Secondary 3 only needs the sin and inverse sin keys; the skill is substituting into n equals sin i over sin r. Without the keys, do the angle comparison and ray diagrams now and leave the calculation for Secondary 4.

Can 45 minutes cover both reflection and refraction?

It can. Each script runs about five minutes and the rest is drawing and practice. Drop a question type rather than the time students spend drawing the normal themselves.

Is there practice material?

Yes. The homework rows link to the Light and Waves chapters in the physics index, which use the same question types and list the question numbers.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.