Category: light

Reducing Sky Glow

Stargazing is awesome! But sometimes, the night sky is not visible due to light pollution. In cities like Nashville, a common type of light pollution is “sky glow”.
Sky glow is the brightening of the entire night sky, especially in populated areas. The light pollution around Nashville inhibits our view of zodiacal light, airglow, and many of the Messier objects.

Bortle Dark-Sky Scale by Skyglowproject

How can we reduce sky glow? One method for cities to reduce skyglow is to reduce uplighting and excess / wasteful light through shielding or cutoff lights. For other tips, check out this blog by Mara Bermudez.

Shielded Lights

Light fixtures prevent light from immediately scattering in the atmosphere. Many fixtures also aim the light towards the ground, reducing glare and providing better ground visibility. Light shielding also reduces unwanted lighting on other people’s property, called light trespass.

Cutoff Lights

The Main Types of Cutoff Lights. by The Nguyen Manh, slide 26.

Cutoff lights focus light towards the ground similarly to shielding fixtures. The amount of cutoff determines how much light gets distributed at or above the horizontal. Cutoff lights have many benefits, including focused light for better surface visibility, and less uplighting, which is wasteful and the excess light scatters in the atmosphere.

In Nashville

Broadway night life in downtown Nashville. by John Russell/Vanderbilt University

Many of the street lights at Vanderbilt University are non-cutoff, as shown in this picture on the Vanderbilt admissions page of downtown Nashville. This type of light pollution contributes to the skyglow around Nashville.

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Why is the Sky Blue?

Schematic animation of a continuous beam of light being dispersed by a prism. The white beam represents many wavelengths of visible light, of which 7 are shown, as they travel through a vacuum with equal speeds c. The prism causes the light to slow down, which bends its path by the process of refraction. This effect occurs more strongly in the shorter wavelengths (violet end) than in the longer wavelengths (red end), thereby dispersing the constituents. As exiting the prism, each component returns to the same original speed and is refracted again.
Light Dispersion Conceptual Waves by
Lucas V. Barbosa. 2007.

Some Background

In the image above, a beam of light passes through a medium. The medium slows down the light and causes it to refract. And the degree of refraction is dependent on the wavelength of light: shorter wavelength light will slow down more and therefore have a greater angle of refraction. See Cauchy’s equation below for the inverse relationship between refractive index and wavelength of light in a transparent medium.


B + \frac{C}{\gamma^{2}}

Our Sun emits a continuous spectrum of light, called black body radiation, which is predictable by its temperature. For the purposes of clarity, we will assume that the Sun emits all the visible wavelengths of light and they reach Earth’s atmosphere.

Why does the sky appear blue?

The shorter wavelength blue light scatters in Earth’s atmosphere more than the longer wavelength red light. Thus, the sky appears blue. And when the Sun’s light travels a long distance to reach us, such as during a sunrise or a sunset, the sky will appear yellow, orange, or red. This phenomenon also explains why a lunar eclipse appears reddish in color, since red light scatters the least as Earth reflects light into space.

Since violet is the shortest wavelength, the sky should appear violet, right?

The sky does appear violet… but not to humans.

The human eye uses three different types of cones to view color. About 64% of these cones are sensitive to red light, about 32% are sensitive to green light, and about 2% are sensitive to blue light. The sensitivity of the cones tend to be similar, despite the disparity of blue cones.

Simplified human cone response curves. Curves show blue, green, and red sensitivities.
Simplified human cones response curve. based on Dicklyon, which is based on Stockman, MacLeon and Johnson. by Vanessaezekowitz. 2007

When the light from the atmosphere reaches our eyes, the blue-sensitive cones are stimulated the most, with a small amount of stimulation to the green- and red-sensitive cones. This mixed hue actually creates the same cone response as “pure” blue and white light.

In the same vein, animals have varied abilities to see color. Many animals only have two cones instead of three. And some animals can see wavelengths invisible to humans. For instance, the honeybee can see ultraviolet light and discerns UV patterns on flowers, which facilitates gathering nectar.

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How Much Does Light Weigh?

Light is made of photons. And photons are massless particles, which means they have no invariant/resting mass. Therefore, light has no mass and no weight. End of story, right?

The Force of Light

Did you know that light exerts pressure on objects? This force can even increase an objects weight, albeit to a small degree. For instance, Vsauce explains how much a landmass might weigh covered in sunlight (for the city of Chicago, sunlight only adds 300 lbs).

Michael Stevens from Vsauce describes the weight of light. Octover 7, 2012. Youtube. For the section we discuss, please watch up to 0:56.

Although insignificant on a small scale, scientists must account for this force – called radiation pressure or solar radiation pressure – in planing space missions.

Solar radiation pressure plays a role in the formation of galaxies, stars and clusters, and solar/planetary systems. Additionally, solar radiation shapes the tails of comets.

A Common Misconception

Light carries energy and momentum. We know that energy, momentum and mass are related. Can we assume that light also has mass?

E = mc^{2}

This misconception stems from Einstein’s mass-energy equivalence formula. Einstein proposed that an object with energy has an equivalent amount of mass. Einstein’s formula only applies to objects with invariant/resting mass. Since photons have no resting mass, we have to use a different formula:

E = pc

where p is the momentum of the particle. Therefore, we can observe momentum and energy for massless particles.

tl;dr Light does not have weight or mass. Light can push an object or increase its weight, to a minimal degree.

We Weigh Less in the Dark, technically

Do we actually weigh more in sunlight? Functionally, no. But, we can still estimate an upper bound.

First, solar radiation pressure is applied to objects in the direction of sunlight. For this problem, let us pretend that we are shaped like rectangular solar panels.

Second, we learned from Vsauce that light exerts a force of pressure on the surface of Earth of about 1e-9 lbs per square inch. Considering the average human has a surface area of 1.9 meters squared, we can calculate the following:


\frac{1.0*10^{-9} lbs}{in^{2}} * \frac{1550 in^{2}}{m^{2}}\ * 1.9m^{2}

= 3.0 * 10^{-6}lbs

So the next time you weigh yourself, turn off the lights. You might not notice a difference, but you just shaved a couple millionths of a pound.

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