S3 Science | Reflection and Refraction: Mirrors, Refractive Index and Lens Images

2026-11-12

S3 is where students first calculate with light. Most mistakes come from measuring the angle in the wrong place and from calling a virtual image real. This lesson fixes three rules with one mirror, one glass block and one lens.

Objectives — By the end of 45 minutes students can draw the normal, mark the angle of incidence and apply i = r, calculate a refractive index with n = sin i / sin r, use 1/f = 1/u + 1/v to find an image distance and decide whether the image is real or virtual, and explain why a plane mirror image is the same distance behind the glass as the object is in front.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Measuring angles	Is the angle of incidence measured from the normal or from the mirror surface?	Most say the surface; correct it at once
Proportion	An object is 30 cm in front of a mirror. Where is the image?	They say 30 cm but cannot say why
Fractions	What is 1/15 minus 1/30?	Often written as 1/15; common denominators needed

02 Lesson flow (45)

Time	Stage	Tutor does	Student does
0-4 min	One mirror, one question	Shine a torch at a plane mirror and ask where the light goes; sketch the incident and reflected rays	Describe the path of the light aloud
4-14 min	The law of reflection	Draw the normal first, mark i and r, show i = r with 35 degrees, stress that the normal is not the mirror	Mark i and r and calculate 35 degrees
14-24 min	Plane mirror images	Object at 30 cm gives an image 30 cm behind; explain virtual, same size, laterally inverted; ask for the object-image distance	Draw the image and calculate 2d
24-34 min	Refraction and n	Air to glass with i = 45 and r = 30 gives n = 1.414; then dense to less dense bends away and the critical angle is 45 degrees	Substitute into the formula and state the bend direction
34-41 min	Five steps for lenses	Write 1/f = 1/u + 1/v; f = 15, u = 30 gives v = 30 (magnification 1); u = 20 gives v = 60 (magnification 3)	Do one calculation alone and state real or virtual
41-45 min	Exit check	Three quick oral questions: where the angle is measured, the image distance, how n is found	Answer and self-correct

03 Teaching script (read it out)

Scene 1: Minutes 4-14 | The law of reflection

Draw a horizontal mirror and add a dashed vertical normal. Draw the incident ray on the left, measure its angle from the normal and write it as i, then draw the reflected ray and measure r. Repeat three times with 20, 35 and 50 degrees, saying “i equals r” each time. Then draw a ray almost flat against the mirror and ask whether its angle of incidence is large or small: a ray close to the surface has an angle close to 90 degrees from the normal, and this is where students measure the wrong angle. Close with: normal first, then the angle, both sides equal.

Students draw the normal unprompted, label i and r correctly and get i = r three times

Scene 2: Minutes 14-24 | Plane mirror images

Draw an arrow as the object on the left and a dashed arrow at the symmetric position on the right. Measure 30 cm in front and 30 cm behind so students read the two numbers themselves. State three properties: virtual because no light actually crosses behind the mirror, same size, laterally inverted. Ask for the distance between object and image and show 30 + 30 = 60 cm, warning that students often answer with the image distance only. Finally move the mirror 10 cm towards the object and ask what happens: the separation becomes 2 x 20 = 40 cm.

Students draw a symmetric virtual image and recalculate the separation after the mirror moves

Scene 3: Minutes 24-34 | Refractive index and total internal reflection

Place a glass block on paper, extend the incident ray straight through as a dashed line, then draw the actual refracted ray so the bend towards the normal is visible. Record i = 45 and r = 30 and substitute step by step: sin 45 = 0.707, sin 30 = 0.500, n = 1.414. Draw two consequences: glass to air bends away from the normal, and beyond a certain angle no light leaves at all. The critical angle satisfies sin C = 1/n, which is 45 degrees for this glass. Draw i = 50 and ask where the light goes: it stays inside and reflects.

Students substitute into the formula and explain why a steep ray from glass to air produces no refracted ray

Scene 4: Minutes 34-45 | Lenses and wrap-up

Write 1/f = 1/u + 1/v. With f = 15 and u = 30: 1/v = 1/15 – 1/30 = 1/30, so v = 30 cm and magnification is 1. With u = 20: v = 60 cm and magnification 3, noting that u closer to f gives a larger v. Then take u = 10, smaller than f: 1/v = 1/10 – 1/10 gives zero, so recompute as 1/v = 1/f – 1/u to obtain minus 30 cm, meaning a virtual image on the same side, magnified 3 times. Close with the five steps: list knowns, unify units, substitute, judge the sign, state real or virtual.

Students complete one lens calculation using the five steps and explain a negative image distance

04 Question types and worked steps

Type	Example	Steps	Answer
Reflected angle	The incident ray makes 55 degrees with the mirror. Find the angle of reflection.	1. Draw the normal. 2. Angle of incidence = 90 – 55 = 35. 3. Angle of reflection = 35. 4. State that angles are measured from the normal.	35 degrees from the normal
Plane mirror separation	An object is 45 cm from a plane mirror. How far apart are object and image?	1. Image distance equals object distance, 45 cm. 2. Separation = 45 + 45. 3. Keep the unit as cm.	90 cm apart
Refractive index	Light passes from air into a liquid with i = 60 and r = 25. Find n.	1. n = sin i / sin r. 2. 0.866 / 0.423. 3. Give three significant figures.	n is about 2.05
Convex lens image	f = 10 cm and u = 15 cm. Find the image distance and magnification.	1. 1/v = 1/10 – 1/15 = 1/30. 2. v = 30 cm. 3. Magnification 30/15 = 2. 4. Real, inverted.	v = 30 cm, magnification 2, real and inverted

Sanity check (teach this): Three sanity checks: the angle of reflection must equal the angle of incidence and can never exceed 90 degrees; a refractive index is always greater than 1 because light is fastest in a vacuum; for a convex lens, u greater than f gives a positive image distance (real) while u less than f gives a negative one (virtual), so a negative answer is not an error.

05 Independent practice (3 questions)

Question	Answer	What to watch
The incident ray makes 70 degrees with the mirror. Find i and r.	i = 20 degrees, r = 20 degrees	Did they subtract from 90 first
Object at 25 cm; the mirror moves 5 cm towards it. Find the new object-image separation.	New object distance 20 cm, separation 40 cm	Did they count the movement twice
f = 8 cm, u = 24 cm. Find v and the magnification.	v = 12 cm, magnification 0.5, real and inverted	Did 1/8 become 8 by mistake

06 Common mistakes and what to say

Mistake	Student writes	Say this
Measuring from the mirror	Using the angle between ray and mirror as the angle of incidence	The angle of incidence is measured from the normal, not the mirror. You measured 55, so the real angle is 35, and the reflected angle is then wrong too.
Calling a virtual image real	Saying light meets behind a plane mirror	No light goes behind the mirror; it reflects into your eye. That is why we call it virtual and why no screen can catch it.
Inverting the refractive index	Writing n = sin r / sin i and getting 0.707	Light slows down in glass, so n must be greater than 1. Getting 0.707 means the fraction is upside down.
Ignoring the sign in the lens equation	u = 10 with f = 15 gives v = 30, so the image is called real	When u is less than f the image distance comes out negative, which means virtual, same side, magnified. Never drop the minus sign.

07 Homework (paper, section and question numbers)

Secondary Science hub — Other chapters in the same subject; finish the three practice items first
Same subject: S2 electricity and circuits — Optics is followed by electricity; revise current and voltage measurement
Same subject: S3 forces and simple machines — Another formula-based lesson; compare the five-step working

08 Exit check (last 5 minutes)

Question	Answer	Decision
Which line do we measure the angle of incidence from?	The normal, perpendicular to the mirror	If they say the surface, redraw the normal
Object at 30 cm: how far apart are object and image?	60 cm, with the image 30 cm behind the mirror	If they answer 30 cm, redo 2d
Why is n always greater than 1?	Light slows down in any medium compared with a vacuum	If unclear, repeat the glass block demonstration

09 Tutor FAQ

Should students memorise the formulas?

Yes, but only after they can name each symbol. u is object distance, v is image distance, f is focal length; confusion over symbols causes most substitution errors.

What if there is no equipment?

A torch and a mirror are enough. Without them, draw on the board and always measure from the normal; the routine matters more than the apparatus.

How far should negative image distances go?

As far as “negative means virtual and on the same side”. S3 does not need the full sign convention for magnification.

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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.