DSE Physics: Reflection, Refraction and Lenses Teaching Guide (S4 Optics, Critical Angle, Magnification)

2026-11-04

Tutor lesson-prep guides · Secondary Physics

Students can recite that the angle of incidence equals the angle of reflection, then draw all three special rays as one. This lesson uses a pencil, a glass of water and one formula to turn optics into predictable geometry.

Objectives — Students can: (1) compute critical angles for water (n = 1.33) and glass (n = 1.5), giving 48.8 and 41.8 degrees; (2) use 1 over f equals 1 over u plus 1 over v to find an image distance of 30 cm and magnification of -2 for an object at 15 cm; (3) distinguish real from virtual images and explain why an object at 8 cm gives -40 cm and magnification 5.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Refraction	Which way does light bend entering water from air?	Towards the normal
Frequency and speed	Does frequency change when light enters a medium?	No, speed and wavelength change
Reciprocals	What is one tenth plus one fifteenth?	One sixth
Similar triangles	Which two ratios give magnification?	Image height over object height, or image distance over object distance

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Hook: a pencil in water	A glass of water and a pencil	Say light changes direction at the surface
5-15 min	Reflection and refraction laws	Normal, angle of incidence, angle of refraction	Write n1 sin theta1 equals n2 sin theta2
15-30 min	Critical angle	From n equals 1.5, sin C equals 1 over n	Obtain C equals 41.8 degrees
30-42 min	Lens formula	f equals 10 cm, u equals 15 cm	Obtain v equals 30 cm and m equals -2
42-54 min	Students draw ray diagrams	Each of the three special rays once	Solid lines for real, dashed for virtual
54-60 min	Exit check	Two oral questions	Correct answers before leaving

03 Teaching script (read it out)

Scene 1: Use a pencil to build the idea of a boundary

Put a pencil in a glass of water and look from the side: it appears broken at the surface. Ask students to describe what they see, then draw the normal and show that light leaving water bends away from it, so the eye projecting straight back misplaces the pencil. Do not rush to the formula. Refraction happens because speed differs across the boundary. Then write n1 sin theta1 equals n2 sin theta2 and define refractive index as the ratio of the speed of light in vacuum to that in the medium.

Ask whether light bends towards or away from the normal leaving water.

Scene 2: The critical angle is a threshold

Setting the refracted angle to 90 degrees gives sin C equals 1 over n. Glass with n equals 1.5 gives 41.8 degrees and water with n equals 1.33 gives 48.8. Demonstrate that beyond this angle no light leaves, ideally with a laser and a water bottle. Students ask why the critical angle exists only from dense to less dense; the formula answers it, because n1 below n2 makes sin C exceed 1 and no solution exists. Mention optical fibres and endoscopes after the derivation, not before, so the application has something to hang on.

Ask students which direction the critical angle applies to.

Scene 3: The lens formula is only a reciprocal relation

1 over f equals 1 over u plus 1 over v. With f equals 10 cm and u equals 15 cm, v is 30 cm and m equals minus v over u equals -2: a real, inverted image. Move the object to 8 cm, inside the focal length, and v becomes -40 cm, negative, which means a virtual image with m equals 5, upright and magnified: that is a magnifying glass. The teaching point is the sign convention. Real is positive, virtual negative, and a negative m means inverted. Insist on one convention so the answer and the diagram agree.

Ask students what a negative v means.

04 Question types and worked steps

Type	Example	Steps	Answer
Refraction sum	Light enters glass at 45 degrees	(1) use n1 sin theta1 equals n2 sin theta2 (2) solve for sin theta2 (3) inverse sine	About 28.1 degrees
Critical angle	Glass with n equals 1.5	(1) sin C equals 1 over n (2) inverse sine (3) dense to less dense only	C equals 41.8 degrees
Lens imaging	f equals 10 and u equals 15 cm	(1) substitute (2) solve for v (3) decide real or virtual	v equals 30 cm, real and inverted
Magnification	Same setup	(1) m equals minus v over u (2) sign means orientation (3) magnitude means size	m equals -2

Sanity check (teach this): Checks: sin C for glass is 1 over 1.50, giving 41.8 degrees; for water 48.8 degrees; with f equals 10 cm and u equals 15 cm, v is 30 cm and m is -2; at 8 cm, v is -40 cm and m is 5. (1) Angles are always measured from the normal, never the surface. (2) Refractive index exceeds 1, so any sine above 1 signals the wrong direction. (3) A negative v is a virtual image, not the absence of an image. (4) A negative magnification means inverted, not smaller. (5) Virtual rays are dashed, real rays solid.

05 Independent practice (3 questions)

Question	Answer	What to watch
Compute a critical angle	A medium with n equals 1.6	About 38.7 degrees
Compute an image	f equals 10 cm, u equals 20 cm	v equals 20 cm, m equals minus 1
Draw a ray diagram	A convex lens with u equals 8 cm	Dashed extensions, image on the same side

06 Common mistakes and what to say

Mistake	Student writes	Say this
Measuring from the surface	“The angle of incidence is 45” while the diagram measures from the water line	“Angles are always measured from the normal, never from the surface.”
Total internal reflection the wrong way	“Light going from air into glass totally reflects”	“A critical angle only exists from dense to less dense. The reverse cannot happen.”
Treating negative v as no image	“v is minus 40, so there is no image”	“A negative v is a virtual image on the same side: that is how a magnifying glass works.”
Reading minus in m as smaller	“m equals minus 2 means half size”	“The minus means inverted; the magnitude 2 means twice as large.”

07 Homework (paper, section and question numbers)

S4 optics chapter — Redo the chapter questions and draw the normal each time
DSE physics paper — Do only the lens questions and label real or virtual
This chapter — With f equals 10 cm, tabulate u of 5, 10, 15, 20 and 30 cm

08 Exit check (last 5 minutes)

Question	Answer	Decision
What is the critical angle formula?	sin C equals 1 over n, dense to less dense only	Answering n means re-teach the derivation
With f equals 10 and u equals 15, where is the image?	30 cm, real and inverted	A negative answer means re-teach the convention
What does m equals minus 2 mean?	Inverted and twice as large	Answering smaller means re-teach

09 Tutor FAQ

Do I need a laser to demonstrate?

It helps, but a glass of water and a pencil already show the displacement.

Must students memorise sign conventions?

Yes: real v positive, virtual negative, inverted m negative. One convention prevents confusion.

Why teach total internal reflection?

It explains optical fibres and endoscopes and appears regularly as an explanation question.

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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.