Science Tip: Big and Blurry, Small and Sharp

I’m sure most of us have seen the type of question below.

It’s not a difficult question but many students get careless with the above when they can’t remember how the proximity of the object from the light source or screen affects its shadow. Here are some simple tips to get it right!

The Reason

Let’s first start with the science of how light and shadows interact. The characteristics of the shadow formed is affected by the distance between the object and the light source, as well as the distance between the object and the screen. This is due to how the light is being blocked and that light travels in a straight line.

Assuming you have a set-up with only one light source in a completely pitch-black room, you would get the following results.

As you can see, the closer the object is to the light source, the more light is blocked, hence the bigger the shadow. A similar thing happens when you move a dollar coin closer and closer to your eye. Eventually, your vision would be totally blocked, even though the size of the coin is constant throughout.

As light is reflected off different surfaces of the room, some of it lands on the shadow. This forms a lighter shade around the shadow, causing it to be blurrier. The closer the object is to the screen, the smaller the shadow formed and the smaller the blurrier region, causing the shadow to be sharper.

The Alternative

For some students, it may be hard to remember how this works, and too cumbersome to draw the diagrams. Here’s an elegant and simple solution that only requires what should be in every student’s pencil case: Two pens and an eraser.

The pens represent the straight light rays from a light source and the eraser is the object. As shown, the closer the eraser is to where the “light rays” join, the more they are forced apart, and the bigger and blurrier the shadow formed.

The Rhyme

“Big and blurry, small and sharp. Big and blurry, small and sharp.” That’s the mantra I get my students to chant while performing the actions shown below.

The idea is simple: If you consider yourself as the light source, the shadow becomes bigger and blurrier as the object moves towards you. The shadow becomes smaller and sharper, when the object moves away from you.

These 2 tips should shed a light on this topic and help your child tackle such questions in the coming exams without a shadow of a doubt!

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