S2 Science | Reflection, Refraction and Dispersion Prep Guide (for tutors) | Measuring from the normal, critical angle, rainbows

2026-11-06

Every student has heard that light bends, yet the moment they draw a ray they measure the angle from the surface and bend the ray away from the normal on the way into water. The definition is not the problem; nobody ever fixed the rule that every angle is measured from the normal.

Objectives — Students can: (1) state the law of reflection using the normal and explain why the image in a plane mirror is as far behind the mirror as the object is in front; (2) decide whether a ray bends towards or away from the normal going between air and water, and calculate the refracted angle for 30 degrees incidence (about 19.5 degrees); (3) state both conditions for total internal reflection and use the critical angle of 41.8 degrees to judge whether light hitting glass at 45 degrees reflects internally.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Measuring angles	Where do you measure an angle from	From the normal, the dashed line perpendicular to the surface
Straight-line travel	How does light travel in one medium	In a straight line, which is why we draw rays
Comparing numbers	Which is larger, 1.50 or 1.33	1.50, so glass is optically denser than water

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	A ray at an angle	Shine a laser into a glass of water and ask where the ray turns	Sketch the three segments they see
8-20 min	Law of reflection	Draw the normal, measure both angles, show image distance equals object distance	Draw one diagram and label both angles
20-32 min	Direction of bending	Air to water bends towards the normal, water to air away; work 30 degrees to 19.5 degrees	State both rules and draw one diagram each
32-44 min	Critical angle	Get 41.8 degrees from n equals 1.50 and test 45 degrees	Judge three angles for total internal reflection
44-56 min	Dispersion and rainbows	Split white light and explain why violet bends most and red least	Rank three colours and give the reason
56-60 min	Exit check	Collect and mark three questions, name the act each error came from	Work independently

03 Teaching script (read it out)

Scene 1: Every angle from the normal

The law of reflection is one sentence: the angle of incidence equals the angle of reflection, and both are measured from the normal. The normal is the dashed line perpendicular to the surface, not the surface itself.
Nine out of ten wrong diagrams measure from the surface: 30 degrees from the surface is really 60 degrees from the normal, and every later step is wrong. So the first move is not the formula, it is the routine: draw the normal first, then mark the angle.
Add one line for plane mirrors: image distance equals object distance. An object 30 cm from the mirror has its image 30 cm behind it, so the two are 60 cm apart, not 30.

Students repeat: every angle is measured from the normal, and the angle of incidence equals the angle of reflection.

Scene 2: Refraction: into a denser medium

Going from air into water or glass means going from a less dense to a more dense medium. Light slows down, the ray bends towards the normal, and the angle of refraction is smaller than the angle of incidence. Leaving water for air, the ray bends away.
The refractive index n is sin i divided by sin r. For glass n equals 1.50: at 30 degrees, sin 30 is 0.5, divided by 1.50 gives 0.333, which is a refracted angle near 19.5 degrees.
Mention the speed in the same breath: in glass light travels at 3 times 10 to the 8 divided by 1.50, which is 2.0 times 10 to the 8 metres per second. Slower is not a metaphor.

Students do: calculate the refracted angle in water (n equals 1.33) for 40 degrees, which is about 28.9 degrees.

Scene 3: Critical angle and dispersion

Total internal reflection needs both conditions: light travels from a denser to a less dense medium, and the angle of incidence is larger than the critical angle. For glass, sin c equals 1 divided by 1.50, giving about 41.8 degrees.
So a ray at 45 degrees inside glass always reflects internally, which is how periscopes and optical fibres work. A ray at 30 degrees never does, because 30 is below 41.8.
Dispersion: white light splits into seven colours in a prism. Violet has the largest refractive index, travels slowest and bends most; red bends least. A rainbow is one refraction, one internal reflection and one more refraction inside water droplets.

Students explain: why violet bends more than red (larger refractive index, slower speed).

04 Question types and worked steps

Type	Example	Steps	Answer
Ray diagram	An object sits 30 cm in front of a plane mirror	Draw the normal, then place the image as far behind	Image 30 cm behind, 60 cm from the object
Refraction calculation	30 degrees from air into glass, n equals 1.50	sin i divided by n gives sin r, then inverse sine	About 19.5 degrees, towards the normal
Total internal reflection	A ray at 45 degrees inside glass	Find the critical angle first, then compare	It reflects internally, 45 is above 41.8
Dispersion ordering	Red, green and violet through a prism	Larger refractive index means more bending	Violet bends most, then green, then red

Sanity check (teach this): Sanity checks: entering water, the refracted angle is always smaller than the angle of incidence; at 0 degrees along the normal the ray does not turn at all; a critical angle only exists going from denser to less dense, so air into glass never reflects totally; the reflected ray leaves on the opposite side of the normal, and the two angles never add to 90 degrees.

05 Independent practice (3 questions)

Question	Answer	What to watch
Mirror distance	An object is 25 cm from a plane mirror. How far from its image	Check that 25 cm is not given as the answer
Refraction in water	40 degrees from air into water, n equals 1.33	Check the ray bends towards the normal, about 28.9 degrees
Three angles	Inside glass at 30, 41.8 and 45 degrees	Check that each angle is compared with the critical angle

06 Common mistakes and what to say

Mistake	Student writes	Say this
Measuring from the surface	Marking 30 degrees on the surface and calling it the angle of incidence	We measure every angle from the normal. Your 30 degrees from the surface is 60 degrees from the normal, so all the later work has to be redone.
Bending the wrong way	Saying light bends away from the normal going into water	Going into water means a denser medium, so it slows down and bends towards the normal. That is why a pencil looks broken in a glass of water.
Total internal reflection anywhere	Claiming light going from air into glass reflects totally	Total internal reflection needs denser to less dense. From air into glass the ray only refracts, however steep it is.
Violet bends least	Saying red bends most so red sits on the outside of a rainbow	Violet has the largest refractive index and bends most. Red appears on the outside because of the viewing angle, not because it bends more.

07 Homework (paper, section and question numbers)

Secondary Physics chapter index — All chapters for this subject
Prep guide master index — Matching chapters in other subjects
Redo this chapter — Recalculate at 50 degrees and re-test total internal reflection

08 Exit check (last 5 minutes)

Question	Answer	Decision
Where is the angle of reflection measured	From the normal	Answering from the surface means act one again
30 degrees into glass, refracted angle	About 19.5 degrees, towards the normal	Answering larger than 30 means act two again
Critical angle of glass	About 41.8 degrees, so 45 reflects internally	Missing the conditions means act three again

09 Tutor FAQ

Do S2 students memorise refractive indices?

Only glass at 1.50 and water at 1.33 are worth remembering; questions supply the rest, and the skill is using sin i over sin r.

What if there is no laser pointer?

A pencil in a glass of water shows refraction clearly, and the side of a filled clear bottle shows total internal reflection.

Do they need to calculate dispersion?

No calculation, but they must say violet has the largest refractive index and bends most, because the next optics chapter reuses it.

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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.