Diffuse reflection: occurs when light rays reflect on a rough surface at many different angles

Our ability to see and make sense of the world with our eyes depends on the reflective properties of light. Without reflection, we would only see luminous objects like the sun, light bulbs, and computer screens.

The light rays that allow us to see non-luminous objects such as our hands, the floor, and the people around us are lit by light rays that travel from a light source to the object and then bounce off the object towards our eyes.

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The light rays that arrive at our eyes after reflecting off of an object behave predictably - consistent with the law of reflection. How we perceive that light is influenced by the characteristics of the object's surface.

One characteristic of a surface that influences what we see is the surface's roughness because there are two general types of reflection: specular and diffuse. The smoother the surface is, the more specular the reflection.

On a very smooth surface, lines normal to neighboring points along that surface are parallel to each other. Since the angle of reflection* depends on the orientation of the normal a the point the light ray hts, a set of parallel incident ray*s encountering a smooth surface reflect so that the angles of reflection are all the same. As a result, the reflected ray*s will retain the same organization present in the incident rays.

Since the reflected rays retain the organization of the incident rays, reflection off of smooth surfaces preserves the organization of the reflected light, allowing us to see images in mirrors.

Diffuse reflection* occurs on rough surfaces. In diffuse reflection, all of the reflected rays still behave in accordance with the law of reflection, but the roughness of the surface results in a variation of the normals along the surface. With this variation, normals at neighboring points are no longer parallel to each other. Since the angle of incidence depends on the normal line at the exact point a ray hits, the incident angles for a set of parallel rays will not be the same, and each reflected ray will have a different angle of reflection. In other words, the rays scatter.

The orientation of the normal lines at neighboring points along a surface differentiates specular from diffuse reflection. If the normal lines are parallel, reflection will be specular. If they are not parallel, reflection will be diffuse. Note this is roughness at the microscopic level. Diffuse reflection occurs on surfaces that are smooth to the touch, such as paper. Specular reflection* occurs on curved surfaces such as a funhouse mirror.

Specular reflection is beneficial. Without it, we would not have cameras or mirrors. However, diffuse reflection is central to our ability to see the world. Aside from the limited number of luminous objects, such as light bulbs and the sun, everything we see around us is visible because of diffuse reflection.

Test your understanding of the concepts covered with the specular vs diffuse reflection problem set.

Video Overview:

Related Content

Reflection of Light:

Objectives:

    1.    Compare regular and diffuse reflections.

    2.    Explain how concave and convex mirrors from images.

    3.    List several uses of concave and convex mirrors.

Resources:

Notes:

Reflection of light:  

When light reaches a solid object one of the things that can happen is that the light can be reflected.  The reflected light can be either partially or completely reflected based on the elasticity of the material.  For this reason, metals make good reflective surfaces.  

Law of reflection:

The law of reflection states that the angle between the incident ray and the normal is equal to the angle between the reflected ray and the normal.

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This law applies to regular, partial and diffuse reflection.

Regular Reflection:

Regular reflection describes reflection off of very smooth surfaces.  It is the type of reflection that can be seen with a mirror or off of a lake on a day without any wind.  Here are a few conditions that are true regarding regular reflection.

Diffuse Reflection:  

When light is incident off of a rough or irregular surface, it can be defined as irregular of diffuse reflection.  This occurs hen you look at the same lake on a windy day.

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Refraction of Light:

Refraction is defined as the bending of light due to a change in its speed.  This is why a straw appears bent in a glass of water or why a swimming pool appears shallower than it really is.  The differences in the position and dimensions are considered apparent because they have not actually changed.  The differences are caused by the changing speed of the light as it transitions to different media.

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Index of Refraction:

The index of refraction is a measure of the relative speed change from air to a different medium.  The greater the change in speed the greater the index of refraction.

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Reflection occurs when light traveling through one material bounces off a different material. The reflected light still travels in a straight line, only in a different direction. The light is reflected at the same angle that it hits the surface. The angle of incidence is equal to the angle of reflection. The angle of incidence is the angle between the incoming light and a line perpendicular to the surface called the normal. The angle of reflection is the angle between the reflected light and the normal. The symbol Ɵ means “angle'' and arrows represent rays of light.

Light reflecting off a smooth surface, where all of the light is reflected in the same direction, is called specular reflection. Along a smooth surface, the normal always points the same way, so all of the light is reflected in the same direction (A on the picture below) and the image that is reflected looks the same as the original image.

The normal at different spots along the rough surface points in different directions, which causes the reflected light to go in different directions. This is called diffuse reflection.  The arrows show in which direction the reflected image will appear when light reflects off a rough surface (B). 

The headlights of a car shine onto the road at night. If the road is dry, the light is diffusely reflected (A), since the pavement is very rough. If the road is wet, the water makes the road surface smoother. There is more specular reflection of the light from the car’s headlights (B). This causes glare (light reflected off the surface like a mirror) that makes it hard for drivers to see.

Refraction

When light traveling through one material reaches a second material, some of the light will be reflected, and some of the light will enter the second material. At the point at which the light enters the second material, the light will bend and travel in a different direction than the incident light. This is called refraction. Refraction happens because the speed of light is different in different materials (though always less than the speed of light in a vacuum).

Think about pushing a shopping cart along cement, and then reaching grass, like in the picture below. It’s harder to push the cart in the grass, so each wheel slows down when it reaches the grass. The wheels on the pavement are still moving faster, so it causes the cart to change directions (in this case, turn to the right).

Index of Refraction

Materials have a property called the index of refraction, which is symbolized using the letter n. The index of refraction of a material is equal to the speed of light in a vacuum, divided by the speed of light in the material. The higher the index of refraction, the slower light travels in that medium. If light is traveling in one material and then refracts in a second material, it will bend towards the normal if the index of refraction of the second material, n2, is greater than the index of refraction of the first material, n1 (the light travels slower in the second material) (n1 < n2) (A). If the second material has a lower index of refraction, the light will bend away from the normal as it travels faster in the second material (n1 > n2) (B). Unlike reflection, the angle of incidence is not equal to the angle of refraction.

The angle of incidence and angle of refraction are mathematically related to the index of refraction of each material through the law of refraction, also called Snell’s Law.

Converging and Diverging Light

A lens is an optical device made of plastic or glass. When light passes through a lens, it can be refracted in predictable ways depending on the shape of the surfaces of the lens. The surface of a lens may be convex (curved outward) or concave (curved inward). When parallel beams of light strike a lens that is convex on both sides (double convex lens) the light is refracted inwards and is said to be converging (A). The beams of light cross at a point called the focal point which is behind the lens (to the right of the lens). When light strikes a lens that is concave on both sides (biconcave lens), the light is refracted outwards and is said to be diverging (B). In this case, the focal point is actually in front of (to the left of) the lens.

Light through a Prism

Light entering a plastic or glass prism (usually a triangular prism), refracts first as it enters the prism and again as it exits the prism. When white light passes through a prism, it is refracted into all of its colours. If you project this light onto a white surface, you see what looks like a rainbow. All of the colours that make up white light are separated at different angles. This is because each individual colour refracts by a different amount through the prism.