SPM Physics Amali - 3 Crosses to Perfection: Mastering Light Ray Tracing in Optics

 

Most students see a ray diagram in a textbook.

Physicists create one.

In this experiment, a ray box and a single-slit plate produce a narrow beam of light. To trace its path accurately, at least three points along the centre of the light ray are marked with crosses.

Once the light source is switched off, a ruler is used to connect the crosses, revealing the exact path of the ray. An arrow is then added to show the direction of travel.

The crosses should be placed at the centre of the beam, not at its edges.

More points generally produce a more accurate ray path.

a modern 6V DC Ray Box

The modern version simplifies everything with a switch:

✅ TOP LIGHT for general illumination

✅ DOWN LIGHT for a narrow experimental ray

Cleaner. Simpler. Faster.

The technology changed. The Physics didn't. From a vintage black Ray Box to a modern 6V light source, every optics experiment still begins with a single beam of light. Not by memorisation, but by OBSERVATION! 

SPM Physics Amali-Apparent Depth & Reflection: What Happens When You Put a Glass Block on Text?

 

This photo was not originally planned as an experiment.

After completing my glass block practical in the Physics laboratory, an idea suddenly came to mind.

I placed the glass block directly on top of one of my AHA! Physics Posters and took this photo.

The result was surprisingly beautiful.

Look carefully at the words underneath the glass block.

The lettering appears to be raised upward, even though the paper is perfectly flat.

What Is Happening?

The thickness of the glass block is called the real depth.

However, when light travels from the lettering through the glass and into the air, refraction occurs.

As a result, your eyes trace the refracted rays backwards and perceive the image to be closer to the surface than it actually is.

This perceived distance is known as the apparent depth.

In Physics:

Apparent Depth < Real Depth

This is why the words seem to "float upwards" inside the glass block.

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Did You Notice Something Else?

Look carefully near the top surface of the glass block.

Besides the raised image, there appears to be a faint duplicate reflection of the dark lettering.

Why?

Because whenever light reaches a glass-air boundary, not all of it is transmitted.

A small fraction is reflected from the surface.

So in this single photograph, two optical phenomena are happening simultaneously:

✅ Refraction (creating apparent depth)

✅ Partial Reflection (creating a faint reflected image)

The glass block is quietly demonstrating both effects at the same time.

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AHA Moment

AHA! The words never moved. The glass simply changed where your eyes think they are. And while refraction lifts the image upward, reflection quietly creates a second one. Not by sight, but by INSIGHT! 

SPM Physics Amali: Refraction & Partial Reflection at Glass-Air Boundary (Amali Optics Real-Lab Demo)

 

I carried out this experiment myself in the Physics laboratory and captured the result as a reference for SPM Physics students.

At first glance, it looks like a simple laser beam passing through a glass block.

Look a little closer.

In a single photograph, you can clearly observe:

✅ Refraction as the laser enters the glass block

✅ Partial Reflection at the glass-air boundary

✅ Refraction Again as the beam exits the glass

That faint upward ray is not a mistake. It is a real reflected ray produced at the boundary, while the rest of the light continues through the glass according to the laws of refraction.

What makes this photo even more special is that the glass block is placed directly on one of my AHA! Crab Optics Posters, turning a textbook diagram into a real laboratory observation.

When Light Travels Along the Normal

In this setup, the incident ray travels parallel to the normal.

This means:

✅ Angle of incidence, i = 0°

✅ Angle of refraction, r = 0°

According to Snell's Law:

When light enters a new medium at 0° incidence, it changes speed but not direction.

The light slows down inside the glass, but because it strikes the surface along the normal, there is no bending.

A Subtle but Important Idea

Although the ray does not bend, something still changes.

✅ Speed decreases inside the glass.

✅ Wavelength decreases.

✅ Frequency remains unchanged.

So even when the path appears unchanged, the Physics is still happening.

AHA! Same glass. Same light. Three different outcomes. Reflection, refraction, or no change in direction at all. Physics is not about memorising rules. It's about knowing when each rule applies.