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Light’s wave nature

We have been using the ray model of light. But consider the light pattern below. It’s impossible – if light only traveled in rays.
Look at the door slit – and then at the pattern of the shadows.

Cindy Cin Yee Law Diffraction of light

Title: Diffraction of Light.  Student: Cindy Cin Yee Law
Holy Trinity School, Richmond Hill, Ontario. Teacher: Nina Dolgovykh

This shows the interference of light, diffracted through a narrow opening between two doors. Sunlight streams in from a large bathroom window behind the double doors on the right. One door is slightly pushed back so that there is a 1 cm space between it and the other door. When the light travels through this opening, it diffracts and spreads out.

At the same time, interference between the emerging waves occurs, causing alternate bands of light and dark. This pattern of dark and light fringes can be seen on the carpet and opposite bedroom doors as shown in the photograph. The interference pattern is so clear that subdivisions of additional bands of light and dark within each bright fringe are also visible. This happens because the source of light is very bright and the projecting screen is far enough from the door slit.

Source: 2009 High School Physics Photo Contest, AAPT

Next example from Matthew Zhang, photography.tutsplus.com

“The tiny aperture hole of a lens, or more specifically the aperture blades, has the effect of bending parallel light rays. Think of an opaque object placed in front of a light source. The mass of the object blocks the light, creating a shadow. Look closely, however, at the edges of that shadow. You may notice that even though the object has a sharp edge, the edges of the shadow are always fuzzy.”

Notice the the difference in sharpness between top of the actual blade and the shadow it casts. I used a photo of a pocketknife to demonstrate the effects of diffraction on a straight line. I took this image in a completely dark room where the only source of illumination was my flash. I also adjusted the contrast on this image in Photoshop to further highlight this effect. Notice that the top of the knife is very straight, and in this image it is rendered very sharply.

However, while looking at the shadow cast by this blade, we notice that the shadow is somewhat fuzzy, even in the presence of a strong, unidirectional light source. This effect that my knife edge has on the light is also observed as light enters a lens, where it interacts with the edges of your aperture blades.

As light bends, it now must travel different distances and begins to interfere with other sources of light created by the aperture blades. This creates brighter areas where the light compounds and dark areas where light is absent.

This is a phenomenon which can be observed not just in light, but in all waves. It is this uneven distribution of light which eventually leads to diffraction.

Diffraction of light around a sharp blade

the shadow is fuzzy, even in the presence of a strong, unidirectional light source

That’s simply impossible – if light traveled in rays (straight lines)

So what is happening? Why are shadows blurry – or making fringes?
You could have a clue about what is happening if you remember our classes on Wave interference and superposition.

Young’s Double slit experiment (1801)

What do we expect to see if we shine light through a single slit?

What do we expect to see if we shine light through a double slit?

We’d expect something like this … but …

Double slit diffraction pattern expectation

(image from http://www.peace-files.com/QF-L-11/01_QF-Double-slit.html)

But that’s not what happens at all. Instead we get this pattern.

Two different sources of light are adding up to make regions of light AND darkness.

Double slit diffraction pattern REALITY

This is a photograph of laser light that went through a double slit.
Light passed through both of the slits – yet when the light hits the wall – we see this interference pattern!

Some areas of light had constructive interference – they become brighter!

Some areas of light had destructive interference – they cancelled out – and created regions of darkness!

Young's Double Slit PASCO

Simplified diagram, as seen from above:

Constructive and destructive interference occurs for water waves, sound waves – and now we know that this is true for light.

Interactive app:

Thomas Young’s Double Slit Experiment: Molecular Expressions


(PDF) How to see that light is a wave – home laboratory of laser optics

Diffraction of light around a razor blade: Harvard Natural Sciences Lecture Demonstrations

YouTube: How to do the “double slit” experiment at home

YouTube: Laser Diffraction and Interference


Young’s experiment – water waves and light waves: Many GIFs


Light waves – like any other waves – can cancel each other out,
or add up to bigger waves, or even curve around an object!



24.1: Waves vs. Particles; Huygens’ Principle and Diffraction
24.2: Huygens’ Principle and the Law of Refraction
24.3: Interference—Young’s Double-Slit Experiment
24.4: The Visible Spectrum and Dispersion
24.5: Diffraction by a Single Slit or Disk
24.6: Diffraction Grating
24.7: The Spectrometer and Spectroscopy
24.8: Interference in Thin Films
24.9: Michelson Interferometer
24.10: Polarization
24.11: Liquid Crystal Displays (LCD)
24.12: Scattering of Light by the Atmosphere

AP PowerPoint: Wave Nature of Light Giancoli

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-PS4-1. Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling within various media. Recognize that electromagnetic waves can travel through empty space (without a medium) as compared to mechanical waves that require a medium

HS-PS4-3. Evaluate the claims, evidence, and reasoning behind the idea that electromagnetic radiation can be described by either a wave model or a particle model, and that for some situations involving resonance, interference, diffraction, refraction, or the photoelectric effect, one model is more useful than the other.

HS-PS4-5. Communicate technical information about how some technological devices use the principles of wave behavior and wave interactions with matter to transmit and capture information and energy. Emphasis is on qualitative information and descriptions. Examples of principles of wave behavior include resonance, photoelectric effect, and constructive and destructive interference.

SAT subject test in Physics: Waves and optics

• General wave properties, such as wave speed, frequency, wavelength, superposition, standing wave diffraction, and Doppler effect
• Reflection and refraction, such as Snell’s law and changes in wavelength and speed
• Ray optics, such as image formation using pinholes, mirrors, and lenses
• Physical optics, such as single-slit diffraction, double-slit interference, polarization, and color

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