# Hands-on science: Is light a wave or a particle?

Scientists argued for two hundred years about whether light was a shower of tiny particles or a series of waves. Then just as the debate was settled, Einstein came along with an answer that would have set Newton’s head spinning.

We decided to explore the properties of waves and light for ourselves.

## What happens when waves meet an obstacle?

When waves meet an obstacle, they curve around it.

If light curved around an obstacle, we would expect the obstacle to cast a fuzzy shadow. But anyone who’s played shadow puppets knows that shadows can have fairly sharp edges.

Because of this, sixteenth century scientist Isaac Newton believed that light must be made of millions of tiny particles moving in straight paths.

## Diffraction

What happens when waves pass through a small opening?

The waves spread out as they pass through, as if the opening was the source of the waves.

Seventeenth century scientist Christian Huygens pointed out that light also spreads out through an opening. If you were to put a lamp behind a wall with a small hole in it, light coming through the hole wouldn’t stay in the shape of the hole – it would spread out.

Huygens said that because light diffracts, it must be made of waves. He pointed out two other properties of light that supported his theory.

## Refraction

A pencil placed in a glass of water appears to bend at the water’s surface. This is because light travels more slowly through water than it does through air.

Huygens said that if light waves travel at different speeds through different materials, the change in speed would cause the waves to bend.  We call this apparent bending refraction.

We performed a cool trick to demonstrate refraction. {1 minute video below} I should have made it clear in the video that the camera stayed still throughout the demonstration!

## Interference

When two sets of waves cross each other, they interact in an interesting way. In some places they cancel each other out, while in other places they add to each other and create a stronger wave. This phenomenon is called interference.

We created two sets of waves in our wave tank. (We would have observed a larger interference pattern in a bigger tank.)

In 1801 Thomas Young proved that light also produces interference patterns.

You can observe light interference patterns by looking at a source of light between two pencils.

When the pencils are almost touching, you can see a vertical pattern of light and dark lines. The dark lines are where the light waves are cancelling each other out.

Thomas Young was the first person to calculate the size of light waves. His measurements explained why light diffraction is so difficult to see – light waves are so small that that can only bend around the tiniest of obstacles.

## Light as both wave and particle?

By the 1800’s scientists were sure that light was made of waves. But in 1900 the particle theory reappeared!

Albert Einstein and Max Planck showed that light sometimes behaves like a wave, but sometimes acts like a particle. Their discoveries led to the branch of science known as quantum physics.

## Resources

We’ve been reading aloud Waves: Principles of Light, Electricity and Magnetism, a wonderful living book I highly recommend. We got most of our experiments from this book.

We read about Einstein and Planck’s fascinating discoveries last term in the Uncle Albert books.

Veritasium has a great demonstration of wave interference, and recreates the original double slit experiment which ‘proved’ (for a while!) that light was a wave:

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Science Sunday at All Things Beautiful

Weekly Wrap-Up at Weird Unsocialized Homeschoolers

## 22 thoughts on “Hands-on science: Is light a wave or a particle?”

1. The video is very cool! The narrator’s nice, posh accent helps too! 😉 Goes to show that our senses often play tricks on us that we may not even be aware of, that is, until we learn the scientific explanation behind them! Thanks for sharing your experiments. The photos showing the waves are very well taken too. They really explain your points very clearly!

1. Thank you, Hwee! I was blown away by the refraction trick. Light is very cool, isn’t it? We’re having a fab time in Spain, thanks. I must write about it soon!

1. Are you guys back yet? It’s been a long while since you’ve written, it seems. You must be having a great time! 🙂

1. Hello Hwee – How lovely that you are thinking of us! We are halfway through our adventure. Learning lots and having a ball, but not being very good about blogging! I shall pull together some pics of our time so far and get something up soon. 🙂

2. Very cool hands-on science lesson! 🙂 I’ll be sharing it with my son for sure. And happy trails on your spectacular trip! We currently live in Lima, Peru and I love how my kids have been immersed in the culture and Spanish language. You’ll create memories to last their lifetime.

1. Thank you, Camie! Lima – wow. Your children are having such a culturally rich childhood. I’m not sure how much cultural immersion is possible in just one month, but it is certainly more interesting than staying home! And after ten days here, my kids at least have a better Spanish accent than me! 🙂

3. Lucinda,

What a great video! I did enjoy hearing your voice and joining in on your experiment.

I hope you’re not a last minute packer. If I got a message telling me my departure time had been brought forward, I’d panic, and then start throwing things into bags at once. I hope you have a wonderful Spanish adventure. Please blog about it when you get back!

Take care and enjoy!

1. Hello Sue, I recorded the video clip last year, and thought it fit in well when I came to do this post. I didn’t realise I hadn’t posted anything with my voice in before! Perhaps that’s a good thing – one of the things I admire most about you is how you just jump in and do things. Thank you for your encouragement, it’s much appreciated. 🙂

We are having a very interesting time here in Spain. If unschooling is learning from everyday life, then everyday life in another country and in another language takes learning to another realm altogether! This afternoon we went on a guided tour of a bullring! I will definitely put together a couple of posts about our trip soon.

4. JEWEL says:

Nice video,got information regarding light and waves play..you and your team explained and demonstrated all in a very simple way and easy to catch..
hope more interesting knowledge session will come soon..

1. Thank you very much, Jewel. Optics can be a very abstract area of science, so I was pleased to come across a way to make it more hands-on.

5. Thanks for the great science lesson – sometimes I feel like I’m getting the pleasure of being homeschooled myself by all the wonderful, informative posts I’m reading! And now I want to show the girls the coin trick – though I’m not sure they’re going to think it’s quite as interesting or “cool” as I did 🙂

1. I know what you mean, Marie – I’m always far more excited by our science experiments than C or J. Perhaps they’ll get it when they share the wonders of the universe with their own children! 🙂
Thank you – we are enjoying our adventure very much!

6. This is a season of science. So many blogs I’ve visited are giving me great project ideas for my 6th grader. Thanks for sharing. I’m visiting from Weekly Wrap Up.

1. Thanks for stopping by, Nita. There are so many fun hands-on science projects out there for little ones, but I think by the time kids are around 6th grade their level of understanding increases and so things can get a lot more interesting. And yet they don’t have to worry too much about exams yet. It’s a fun time.

7. Apparently I’ve had this post open all week long and read it several days ago, but never commented. Now we see how the fog of allergies has affected me.

Brilliant activities, and a great way to simulate how light moves and acts. I loved every single activity.

1. Thanks Tica. I hope the allergy fog has lifted!

8. You sound just like I imagined you would (and very much like your daughter!). These are all great activities. T loves the Uncle Albert books. I just don’t remember physics being this interesting when I was at school!

1. It definitely wasn’t at my school, Claire. All I remember before I gave it up was spending what felt like weeks measuring something with a piece of copper wire. There must have been more to it than that, but it’s what sticks in my mind!

9. Lucinda, I just wanted to give you the heads up that over the next few weeks I will be giving away a lot of the living maths type books. You probably have them all already, but if you don’t feel free to drop me line and let me know the ones you would like (I post the ones I’m giving away during the week on the Monday).
I hope you are all having amazing adventures in Spain and we are all looking forward to an update (hint, hint….)

1. Ooh you’re a star – thank you, I’ll be on the lookout!

And thanks for the encouragement to blog about our Spanish adventures … I’m so busy processing it all I’m not sure where to start! I will, though 🙂