Optics Mcat

And in a lens, a virtual image is on the same side. Now it’s important for you to remember the spherical mirror equation, or 1 over the object distance plus 1 over the image distance, equals 1 over the focal length, which is the same as 2 over the radius of curvature. Now the thin spherical lens equation is also equal to 1/o +1/i = 1/f = 2/r. These two use the same equation.

Optics Mcat

Now let’s transition to Optics. In optics, we’re gonna be dealing with objects and their images. We will have both real images and virtual images. Now a real image is the result of actual light passing through it. In virtual images, your eye extrapolates light passing though the image. Now in a mirror, the real image is on the same side as the object.

So if we had a mirror and an object, the image that’s real would be on the same side of the mirror. In the lens, the opposite is true. If we have our object, the image will be on the opposite side of the lens. For virtual images, the opposite is true. In a mirror, a virtual image is on the opposite side.

And in a lens, a virtual image is on the same side. Now it’s important for you to remember the spherical mirror equation, or 1 over the object distance plus 1 over the image distance, equals 1 over the focal length, which is the same as 2 over the radius of curvature. Now the thin spherical lens equation is also equal to 1/o +1/i = 1/f = 2/r. These two use the same equation.

Lastly, a magnification is given by negative image over object. So if the magnification is negative, this means that the image is upside down. If the magnification is between 0 to 1, it’s going to be smaller. And if the magnification is greater than 1, it will be larger. Now mirrors involve principles of reflection. The law of reflection says that light will reflect off a surface in such a manner that the angle of incidence equals the angle of reflection.

So if you pull up a mirror, we draw the normal, we have an incoming light ray, that angle of incidence, theta, equals the angle of reflection, so that is also theta. By geometry, these two angles must also be equal. Now let’s draw the reflection of an arrow in a plane mirror. In a plane mirror, we’re going to find that for the object, the image will be an equal distance but on the opposite side of the mirror.

Is this image therefore real or virtual? This is going to be a virtual image cuz it’s on the opposite side of the mirror. If we were to look at a light ray, it would bounce like this. Our eye would see it and extrapolate light back to this image. What is the focal length of the mirror? For this we’ll want to recall the equation, 1 over object plus 1 over image equals 1 over focal length.

And we said that if this is the object distance and this is the image distance, object is just simply equal to negative image. The two are equal but in opposite directions. So we can then substitute that in. One over -1 plus 1 over i equals 1 over focal length. This tells us that 0 equals 1 over focal length.

A little way to get 0 is for focal length then to go to infinity. 1 over infinity is 0. So the focal length for a plane mirror is infinity. Which essentially makes it an infinitely large spherical mirror, which we will now discuss. Now spherical mirrors come in two general types.

The concave or converging mirror, where the reflective surface looks cave-like or converges. And the diverging mirror, a convex mirror, where it bows out. Now we can create a spherical mirror by taking a giant sphere whose inside is lined with silver to make it reflective. And then we essentially just cut out a small portion of it.

Now the distance from the center of the sphere to the wall, is the radius of curvature. This point in the middle is called the center of curvature. And half of the radius of curvature is equal to the focal length. And we want to remember these equations. 1/o +1/i =1/f, which is the same thing as 2/r.

And magnification equals -i/o.

Light and Optics for the MCAT: Everything You Need to Know

Learn key MCAT concepts about light and optics, plus practice questions and answers

(Note: This guide is part of our MCAT Physics series.)

Table of Contents

Part 1: Introduction to light and optics

Part 2: Characteristics of light

a) Photons

b) Double- and single-slit experiments

c) Reflection, refraction, and Snell’s law

d) Additional phenomena

Part 3: Mirrors

a) Flat mirrors

b) Spherical mirrors

Part 4: Thin lenses

a) Convex and concave lenses

b) Combining lenses

Part 5: High-yield terms and equations

Part 6: Passage-based questions and answers

Part 7: Standalone questions and answers

Part 1: Introduction to light and optics

In this guide on light and optics, we will study the characteristics of light: including its trajectory and propagation. Light and optics are important for light-related reactions and many precision instruments you might find in a lab. For this reason, it is considered to be a medium-yield topic on the MCAT. In all likelihood, when it does come up on the MCAT, it will be in a strictly biological context as opposed to the physics and mathematical contexts in the sections below. However, understanding the fundamentals of this field will make understanding light and optics in relation to biology much smoother.

Below, the most important terms are in bold font. Be sure to understand these terms and use them to create your own examples. At the end of this guide, you will also find an MCAT-style practice passage and standalone questions. Practicing with these questions will not only test your knowledge of light and optics but also show you how the AAMC likes to ask questions.

Let’s get started!

Part 2: Characteristics of light

Light is an electromagnetic wave of any wavelength. The electromagnetic spectrum—and therefore, what can be categorized as light waves—includes waves of all wavelengths, from radio frequencies to x-rays. Visible light, the light that makes up all the colors you see, only includes light in a small range of wavelengths (about 400-700 nanometers).

a) Photons

Recall that an electromagnetic wave is a wave composed of perpendicularly oscillating electric and magnetic fields. The two fields interact with each other and propagate light in the direction perpendicular to both oscillations.

Since the motion of light is driven by its very own waves, we say that light is “self-propagating.” Light also has a characteristic speed of propagation. The speed of light, often abbreviated as c, is equal to about 3 x 10 8 meters/second when traveling through a vacuum. The speed of light is subject to change as light travels through different materials, or media. (For the purposes of the MCAT, it’s sufficient to assume that the speed of light through a gas is the same value as the speed of light through a vacuum.)

However—perhaps the biggest “however” in physics—light can also be understood as a particle instead of a wave. A photon is a massless particle that represents a discrete unit (or a “quantum”) of light. We will use photons as a representation of light when considering the energy absorbed from light striking an object.
When objects absorb energy from incident light, they can only do so in discrete amounts. This discretized unit is called the photon energy. It is proportional to the frequency of light.

Note that frequency and wavelength are inversely proportional: or that

Thus, light with high-frequency wavelengths has more energy than light with low-frequency wavelengths.

If the wave-particle duality of light doesn’t sit well with you, that’s completely fine! It is a debate that continues to puzzle many physicists today. This information is simply meant to prepare you for the different representations of light that will come later. Some applications will treat light as a wave, and others as a linearly moving particle.

b) Double- and single-slit experiments

All forms of waves share some similarities in their behavior. Interference occurs when two waves collide and combine. Constructive interference occurs when waves are in phase, and sum to make waves with a larger amplitude. Destructive interference occurs when waves are out of phase and cancel each other out to form a wave with zero amplitude. (For more information on this, be sure to refer to our guide on waves and sound.)

As light can be considered as a wave, they also exhibit interference. This phenomenon was first observed during Thomas Young’s double-slit experiment. The results of this experiment led scientists to believe that light behaves as a wave.

In the double-slit experiment, light is shone through two narrow slits placed close together. After passing through the slits, the light strikes a wall and shows alternating bright spots and shadows—a pattern indicative of interference. The “maxima,” or bright spots, are areas of constructive interference. The “minima,” or shadows, are areas of destructive interference.

The following equation determines where these maxima and minima will occur:

Take a moment to match the different terms in the equation to the image above it. While you won’t be expected to apply this equation on the MCAT, it’s useful to understand each variable.

Note that for any interference to occur, the light sources must be coherent, meaning the waves are in-phase. This is satisfied in the double-slit experiment because the light coming through each slit is generated by the same source and therefore must maintain the same phase relationship and amplitude. The other condition for interference is that the light must be monochromatic, meaning it consists of only one frequency or color.

What happens when we change the orientation of these two slits? A screen with many slits evenly spaced and placed close together is called a diffraction grating. The bright spots and dark spots from a diffraction grating are more intense, and the transition between them is more abrupt. The bright spots are relatively narrow, while the dark spots are wider.

In contrast to the double-slit experiment, in the single-slit experiment, light is shone through a single slit. Since there is only one slit, only one beam of light is allowed through. The waves of light passing through this slit are still able to constructively and destructively interfere with each other. The resulting diffraction pattern from a single slit appears to be more generally dispersed than the pattern resulting from a double slit.

c) Reflection, refraction, and Snell’s law

Light interacts with solids and objects in different ways. Reflection occurs when light bounces off of a surface. There are two types of reflection: specular reflection and diffuse reflection.

Specular reflection occurs when light reflects off a smooth surface at a definite angle. The simplest form of specular reflection occurs when light is shone perpendicularly at a surface. After it hits the surface, it is reflected back in the same direction it came from.

When light is shone at an angle, the same concept is at work. To understand this, decompose the initial vector of light into two components. After the reflection, the component pointing perpendicular to the wall reverses its direction. The component pointing parallel to the wall stays the same.