Let the Light Through — Physics (Grade 7)

Define and explain Let the Light Through in your own words
Use key terms such as transparent accurately
Apply what you have learned to new examples and questions
Avoid the common mistakes learners make with this topic

This lesson focuses on Let the Light Through: sorting materials into transparent, translucent and opaque.

Definition: Let the Light Through

Sorting materials into transparent, translucent and opaque.

Key ideas

Some materials let water or light through, others block them

A sponge soaks up water while a plastic bag keeps it out — one is absorbent and the other is waterproof. Clear glass is transparent, frosted glass is translucent, and wood is opaque. These blocking and letting-through properties decide how we use each material.

We choose materials to suit the job

Wellies are made of rubber because rubber is waterproof and bendy. Windows are made of glass because it is transparent and hard. Matching the property to the purpose is the key idea behind materials science, and engineers do it for everything from bridges to lunchboxes.

Key term — transparent: Lets nearly all light through so you can see clearly through it, like clean glass.

Worked example: Let the Light Through

Name one transparent material and one opaque material.

Transparent: glass or clear plastic. Opaque: wood, metal or brick.

Answer: Transparent: glass or clear plastic. Opaque: wood, metal or brick.

Common mistakes

Thinking glass is always transparent Frosted or stained glass is translucent — it lets some light through but you cannot see clearly through it.
Assuming shiny means waterproof Shininess is about light bouncing off the surface; a shiny sponge still soaks up water.

Practice

A material lets some light through but you cannot see through it clearly. What word describes it?

It sits between transparent and opaque.

Translucent.

Sort these into natural and man-made materials: wood, plastic, cotton, glass.

Natural materials come from plants, animals or rocks.

Natural: wood, cotton. Man-made: plastic, glass.

Is a wooden spoon hard or soft? How could you test it?

Try scratching or bending it.

Hard — you cannot dent it with a fingernail or bend it easily.

Why are umbrellas not made of paper?

What happens to paper in rain?

Paper is not waterproof — it soaks up water and tears, so it would fail as an umbrella.

Quick check

Let the Light Through — quick check

Which of these best defines "transparent"?

Lets nearly all light through so you can see clearly through it, like clean glass.

Give one property that makes metal good for saucepans.

Metal conducts heat well, so heat from the cooker spreads quickly through the pan. It is also strong and hard.

Key takeaways

Let the Light Through: sorting materials into transparent, translucent and opaque.
Some materials let water or light through, others block them: A sponge soaks up water while a plastic bag keeps it out — one is absorbent and the other is waterproof.
material: What something is made from, such as wood, metal, plastic, glass, fabric or rock.
Watch out for: thinking glass is always transparent

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