Home / iGCSE Physics (0625) 3.2.3 Thin lens Paper 3 -Exam Style Questions- New Syllabus

iGCSE Physics (0625) 3.2.3 Thin lens Paper 3 -Exam Style Questions- New Syllabus

IGCSE Physics (0625) Paper 3 - All Topics

Question

(a) Fig. 7.1 shows a ray of light striking a plane mirror. The ray is reflected as shown.
The angle of incidence for the ray of light is 40°.
(i) Indicate the angle of reflection by drawing a letter R on Fig. 7.1.
(ii) State the size of the angle of reflection in Fig. 7.1.
(b) An object $O$ is placed to the left of a thin converging lens. $F_1$ is the principal focus on one side of the lens and $F_2$ is the principal focus on the other side of the lens.
Two rays from the top of the object are incident on the lens, as shown in Fig. 7.2.
(i) On Fig. 7.2, locate the image of $O$ by continuing the path of each ray.
(ii) Draw an arrow to represent the image of $O$.
(c) Fig. 7.3 shows a prism producing a spectrum of colours from a narrow beam of white light.
(i) The prism refracts the white light. State the name of the effect that produces a spectrum.
(ii) In the spectrum shown in Fig. 7.3, there are seven colours. List the seven colours in the order they appear between $X$ and $Z$.

Most-appropriate topic codes (Cambridge IGCSE Physics 0625):

• Topic 3.2.1 — Reflection of light (Parts (a))
• Topic 3.2.3 — Thin lenses (Parts (b))
• Topic 3.2.4 — Dispersion of light (Parts (c))

▶️ Answer/Explanation

(a)(i)
For the correct answer:
The angle between the normal (dashed line) and the reflected ray is identified.

The normal is the imaginary dashed line drawn perpendicular to the mirror’s surface at the exact point of incidence. By definition, the angle of reflection is the angle formed strictly between this normal line and the reflected ray, not the angle with the mirror’s surface.

(a)(ii)
For the correct answer:
40°

The fundamental law of reflection states that the angle of incidence is always equal to the angle of reflection. Because the problem states the incident angle is 40°, the reflected angle must also be exactly 40°.

(b)(i)
For the correct answer:
The horizontal ray is drawn to continue through $F_2$, and the ray to the centre is drawn to continue undeviated.

In optics, a ray traveling parallel to the principal axis of a converging lens will always refract to pass through the principal focus ($F_2$) on the opposite side. Conversely, a ray passing directly through the optical centre of a thin lens does not bend and continues in a straight line.

(b)(ii)
For the correct answer:
An image is drawn or identified at the point where the two rays cross.

The top of the real image is located exactly where the refracted rays intersect. To represent this image, you draw a vertical arrow starting from the principal axis and ending at the intersection point, which will point downwards indicating the image is inverted.

(c)(i)
For the correct answer:
Dispersion

As the narrow beam of white light enters the denser glass prism, the different frequencies of light slow down by different amounts. This differential refraction causes the white light to separate into its constituent colours, a physical phenomenon known as dispersion.

(c)(ii)
For the correct answer:
Red, orange, yellow, green, blue, indigo, violet (in the exact order).

Red light has the lowest frequency and is refracted the least, meaning it appears at the top of the spectrum ($X$). Violet light has the highest frequency and is refracted the most, appearing at the bottom ($Z$). The rest of the spectrum naturally falls between them in sequence.

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