Light and Shadows: Complete PSLE Science Guide

Light is one of the most important topics in Singapore Primary Science. It begins in P3 with basic concepts and builds toward more detailed understanding by PSLE. Students must understand how light behaves, what materials do to light, how shadows are formed, and how we can use light in everyday situations. This guide covers everything you need from P3 through to PSLE.

Properties of Light

Light is a form of energy. The most important property you need to know for PSLE is that light travels in straight lines. This is called rectilinear propagation of light. It explains why shadows have sharp edges, why we cannot see around corners, and how periscopes work.

Light travels extremely fast — about 300,000 km per second in a vacuum. For Primary Science purposes, you just need to know that light travels much faster than sound, which is why we see lightning before we hear thunder.

Transparent, Translucent, and Opaque Materials

Different materials interact with light differently. For PSLE, you need to classify materials correctly and explain what happens to light when it meets each type.

How Shadows Are Formed

A shadow forms when an opaque (or translucent) object blocks the path of light. Because light travels in straight lines, it cannot bend around the object — the area behind the object receives no light, forming a dark region called the shadow.

For a shadow to form, three things are needed: a light source, an opaque or translucent object, and a surface (screen) for the shadow to fall on.

Factors That Affect Shadow Size

• Position of the light source — when the light source is closer to the object, the shadow is larger; when it is further away, the shadow is smaller
• Position of the object — when the object is closer to the screen, the shadow is smaller and sharper; when further from the screen, the shadow is larger and blurrier
• Size of the object — a larger object creates a larger shadow
• Angle of the light source — the angle affects the shape of the shadow (e.g. the Sun's angle changes shadow length throughout the day)

Shadows and the Sun

In Singapore, the Sun appears to move across the sky from east to west during the day (due to Earth's rotation). This means shadows change direction and length throughout the day. At noon, when the Sun is highest in the sky, shadows are shortest. In the morning and late afternoon, shadows are longest. This is a favourite PSLE question type — you may be asked to identify what time of day a shadow suggests.

Reflection of Light

When light hits a smooth, shiny surface it bounces back — this is called reflection. Mirrors are the best example of reflection. The angle at which light hits a mirror (angle of incidence) equals the angle at which it bounces off (angle of reflection).

Reflection is why we can see objects that do not emit their own light. The Moon, for example, has no light of its own — we see it because it reflects light from the Sun.

A periscope uses two mirrors positioned at 45° angles to allow someone to see over or around obstacles. Light enters the top, reflects off the first mirror downward, then reflects off the second mirror toward the viewer's eye.

Uses of Light in Daily Life

• Periscopes in submarines use mirrors to see above the water surface
• Road signs and safety clothing use reflective materials to be seen in the dark
• Solar panels convert light energy into electrical energy
• Greenhouses use glass (transparent) to let sunlight in while trapping heat
• Frosted glass (translucent) in bathroom windows provides privacy while letting light through

Model Exam Answers

Question: Ahmad places his hand between a torch and a wall. Explain why a shadow forms on the wall.

Model Answer: Light from the torch travels in straight lines. When the light reaches Ahmad's hand, the hand (being opaque) blocks the light from passing through. Since light cannot bend around the hand, the area behind the hand on the wall receives no light. This dark region is the shadow.

Question: The shadow of a pole is long in the morning but short at noon. Explain why.

Model Answer: In the morning, the Sun is low in the sky. Light hits the pole at a low angle, casting a long shadow. At noon, the Sun is almost directly overhead. Light hits the pole from nearly above, so the shadow is very short. The angle of the light source determines the length of the shadow.