AP Physics 2 : Optics

Study concepts, example questions & explanations for AP Physics 2

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Example Questions

Example Question #1 : Optics

Suppose a student who was farsighted wears glasses that allows him to read at a distance of \(\displaystyle 20cm\) from his eyes to the book. His near-point distance is \(\displaystyle 63cm\). If his glasses are \(\displaystyle 1.5cm\) from his eyes, what is the refractive power of his glasses lenses?

Possible Answers:

\(\displaystyle 3.846D\)

\(\displaystyle 6.346D\)

\(\displaystyle 4.536D\)

\(\displaystyle 1.613D\)

\(\displaystyle 2.342D\)

Correct answer:

\(\displaystyle 3.846D\)

Explanation:

Write the thin-lens equation.

\(\displaystyle \frac{1}{f}= \frac{1}{d_o}+\frac{1}{d_i}\)

\(\displaystyle d_o\) represents the object distance from the lens, subtracting the distance between the student's eyes and the lens.

\(\displaystyle d_o=20cm-1.5cm=18.5cm\)

The image distance is:

\(\displaystyle d_i=1.5cm-63cm=-61.5cm\)

Substitute the givens to the equation.

\(\displaystyle \frac{1}{f}= \frac{1}{18.5}+\frac{1}{-61.5}= 0.037794 cm^{-1}\)

\(\displaystyle f=\frac{1}{0.037794cm}=26.4592cm = 0.26 m\)

Refractive power is the inverse of focal length in \(\displaystyle m\).

\(\displaystyle \frac{1}{0.26m}= 3.846D\)

Example Question #1 : Thin Lens Equation

A flower is placed \(\displaystyle 20cm\) away from a converging lens. An upside down image of the flower is measured as \(\displaystyle 6cm\) tall and at a distance of \(\displaystyle 30cm\) from the lens. Find the focal length of the lens and the real height of the flower.

Possible Answers:

\(\displaystyle f=20cm\)

\(\displaystyle h_{o}=12cm\)

\(\displaystyle f=0.08cm\)

\(\displaystyle h_{o}=6.7cm\)

\(\displaystyle f=12cm\)

\(\displaystyle h_{o}=4cm\)

\(\displaystyle f=30.5cm\)

\(\displaystyle h_{o}=4.5cm\)

Correct answer:

\(\displaystyle f=12cm\)

\(\displaystyle h_{o}=4cm\)

Explanation:

Let the object distance be \(\displaystyle d_{0}\) and the image distance be \(\displaystyle d_{i}\). The object distance is the distance the flower is from the lens. The image distance is the distance the image is located from the lens. The thin lens equation is

\(\displaystyle \frac{1}{f}=\frac{1}{d_{o}}+\frac{1}{d_{i}}\)

\(\displaystyle f=\left ( \frac{1}{d_{o}}+\frac{1}{d_{i}}\right )^{-1}=12cm\)

Alternatively you can use

\(\displaystyle f=\frac{d_{o}d_{i}}{d_{o}+d_{i}}\)

To find the height, the equation for the magnification of an object is given by

\(\displaystyle m=-\frac{d_{i}}{d_{o}}\)

it is also given as

\(\displaystyle m=-\frac{h_{o}}{h_{i}}\)

The negative sign indicates an image that is upside down. Setting these equal gives

\(\displaystyle \frac{d_{i}}{d_{o}}=\frac{h_{i}}{h_{o}} \rightarrow h_{o}=\frac{d_{o}}{d_{i}}h_{i}=4cm\)

Example Question #3 : Optics

You are passing a ray of light through a thin tank of alcohol to determine properties. Assume that the light ray is not effected by the tank's exterior. You find that if a piece of white paper is put underneath the alcohol tank, there is an image on the metal. 

If the top of the image is \(\displaystyle 10cm\) from the center of the tank, and the top of the object is located \(\displaystyle 3mm\) from the center of the tank, what is the focal length of this lens? 

Possible Answers:

\(\displaystyle f= 2.9\)

\(\displaystyle f= .0029\)

\(\displaystyle f=340\)

\(\displaystyle f=5.5\)

Correct answer:

\(\displaystyle f= .0029\)

Explanation:

Using object distance, image distance, and focal length formula:

\(\displaystyle \frac{1}{o}+ \frac{1}{i}= \frac{1}{f}\)

Where \(\displaystyle o\) is object distance, \(\displaystyle i\) is image distance, and \(\displaystyle f\) is focal length. 

Plugging everything in:

\(\displaystyle \frac{1}{.003}+\frac{1}{.1}=\frac{1}{f}\)

Solving for \(\displaystyle f\),

\(\displaystyle f= .0029\)

Example Question #1 : Optics

An object \(\displaystyle 11cm\) tall is \(\displaystyle 9cm\) from a mirror. If the image distance is \(\displaystyle -3cm\) from the mirror, what is the image height?

Possible Answers:

\(\displaystyle 3.67cm\)

\(\displaystyle 11.0cm\)

\(\displaystyle 33.0cm\)

\(\displaystyle -3.67cm\)

\(\displaystyle -33.0cm\)

Correct answer:

\(\displaystyle 3.67cm\)

Explanation:

The Magnification Equation is as follows:

\(\displaystyle M=\frac{h_i}{h_o}=-\frac{d_i}{d_o}\)

The negative on the last part is very important. If you don't include it (and it's easy to forget), you will get the wrong answer.

We'll use the parts without \(\displaystyle M\) for our problem. We want \(\displaystyle h_i\), so we just need to multiply both sides by \(\displaystyle h_o\).

\(\displaystyle h_i=-\frac{d_i}{d_o}\cdot h_o\)

Now, we can plug in our numbers.

\(\displaystyle h_i&=-\frac{d_i}{d_o} \cdot h_o\)

\(\displaystyle h_1=-\frac{-3\cdot 10^{-2}}{9\cdot 10^{-2}}\cdot (11\cdot 10^{-2})\)

\(\displaystyle h_1= 0.0367\)

Therefore, the image height is \(\displaystyle 3.67cm\)

Example Question #2 : Magnification

You have a concave mirror and a candle. The candle is \(\displaystyle 14.7cm\) tall, and the image is \(\displaystyle 21.9cm\) tall. What is the the magnification of the mirror?

Possible Answers:

\(\displaystyle 0.66\)

\(\displaystyle 1.79\)

\(\displaystyle 0.75\)

\(\displaystyle 1.49\)

\(\displaystyle 1.68\)

Correct answer:

\(\displaystyle 1.49\)

Explanation:

The magnification equation is 

\(\displaystyle M=\frac{h_i}{h_o}\)

where \(\displaystyle h_i\) is the height of the image, and \(\displaystyle h_o\) is the height of the object. We can plug these given values into the equation.

\(\displaystyle M&=\frac{h_i}{h_o}\)

\(\displaystyle M=\frac{21.9}{14.7}\)

\(\displaystyle M= 1.49\)

The magnification is a scalar value, so it's unitless. We can verify this by examining what goes into the equation. Both heights have the same units, which cancel, so that leaves us with no units.

Example Question #1 : Optics

The image produced by a concave mirror is at \(\displaystyle -16.2m\), and the magnification is \(\displaystyle 1.79\). What is the object distance?

Possible Answers:

\(\displaystyle 9.05m\)

\(\displaystyle -11.56m\)

\(\displaystyle -9.05m\)

\(\displaystyle 29.00m\)

\(\displaystyle -29.00m\)

Correct answer:

\(\displaystyle 9.05m\)

Explanation:

The equation for magnification is

\(\displaystyle M=\frac{h_i}{h_o}=-\frac{d_i}{d_o}\)

We're given the image distance and the magnification, so we'd use the second equality.

\(\displaystyle M&=-\frac{d_i}{d_o}\)

\(\displaystyle M\cdot d_o&=-d_i\)

\(\displaystyle d_o&=-\frac{d_i}{M}\)

Now, we can plug in our numbers.

\(\displaystyle d_o&=-\frac{d_i}{M}\)

\(\displaystyle d_o= -\frac{-16.2}{1.79}\)

\(\displaystyle d_o= 9.05\)

The negative in front of the equation is very important. If you forget it, the answer will be incorrect.

The distance from the mirror to the object is \(\displaystyle 9.05m\). Note that the object distance is always positive.

Example Question #4 : Magnification

An object of height \(\displaystyle 2.9 cm\) object is placed \(\displaystyle 10 cm\) in front of a convex mirror that has a radius of curvature of \(\displaystyle 67.0 cm\). Determine the magnification of the image.

Possible Answers:

\(\displaystyle 1.88\)

\(\displaystyle .925\)

None of these

\(\displaystyle 6.11\)

\(\displaystyle 1.425\)

Correct answer:

\(\displaystyle 1.425\)

Explanation:

Using the relationship:

\(\displaystyle \frac{1}{p}+\frac{1}{i}=\frac{1}{f}=-\frac{2}{r}\)

Where:

\(\displaystyle p\) is the object distance from the mirror

\(\displaystyle i\) is the image distance from the mirror

\(\displaystyle f\) is the focal length of the mirror

\(\displaystyle r\) is the radius of curvature of the mirror

Plugging in values:

\(\displaystyle \frac{1}{-10}+\frac{1}{i}=\frac{1}{f}=-\frac{2}{67}\)

Solving for \(\displaystyle i\):

\(\displaystyle i=\frac{1}{-\frac{2}{67}+\frac{1}{10}}\)

\(\displaystyle i=14.25cm\)

Using the equation for magnification:

\(\displaystyle M=\frac{h_i}{h_p}=-\frac{i}{p}\)

Where:

\(\displaystyle M\) is magnification

\(\displaystyle M=-\frac{14.25}{-10}\)

\(\displaystyle M=1.425\)

Example Question #5 : Magnification

You are passing a ray of light through a thin tank of alcohol to determine properties. Assume that the light ray is not effected by the tank's exterior. You find that if a piece of white paper is put underneath the alcohol tank, there is an image on the metal. 

Assuming that the image is the size of \(\displaystyle 1.5cm\) and is actually a picture of a tree of size \(\displaystyle 10m\). What can we say about the magnification of this alcohol solution? 

Possible Answers:

\(\displaystyle -1.5*10^{-3}\)

\(\displaystyle 1.5*10^{-3}\)

\(\displaystyle -6.7*10^{2}\)

\(\displaystyle 6.7*10^{2}\)

Correct answer:

\(\displaystyle -1.5*10^{-3}\)

Explanation:

To determine magnification, we simply divide the object length from the image length. 

\(\displaystyle M=-\frac{1.5*10^{-2}m}{10m}=-1.5*10^{-3}\)

The negative sign is used to designate that the image is real. 

Example Question #1 : Optics

Suppose that an object is placed 15cm in front of a concave mirror that has a radius of curvature equal to 20cm. Will the resulting image be upright or inverted? Will it be a real or virtual image?

Possible Answers:

There is no way to determine whether the image is real, virtual, inverted, or upright based on the information given

Inverted and real

Upright and virtual

Inverted and virtual

Upright and real

Correct answer:

Inverted and real

Explanation:

To solve this problem, it is essential to use the mirror equation:

\(\displaystyle \frac{1}{d_{o}}+\frac{1}{d_{i}}=\frac{1}{f}\)

Where:

\(\displaystyle d_{o}\) is the distance the object is from the mirror

\(\displaystyle d_{i}\) is the distance the image is from the mirror

\(\displaystyle f\) is the focal length of the mirror

To calculate the focal length, we'll have to use the radius of curvature:

\(\displaystyle f=\frac{r}{2}=\frac{20cm}{2}=10cm\)

Plugging this value into the top equation and rearranging, we obtain:

\(\displaystyle \frac{1}{d_{i}}=\frac{1}{f}-\frac{1}{d_{o}}=\frac{1}{10cm}-\frac{1}{15cm}=\frac{1}{30cm}\)

Therefore, the image distance is:

\(\displaystyle d_{i}=30cm\)

Since the image distance calculated above is a positive value, this means that the image forms on the same side of the mirror that the reflected light traveled. Thus, the image is real.

To determine whether the image is inverted or upright, we'll need to use the magnification equation:

\(\displaystyle M=\frac{h_{i}}{h_{o}}=-\frac{d_{i}}{d_{o}}\)

\(\displaystyle M=-\frac{30cm}{15cm}=-2\)

The magnification obtained above is a negative number. As a result, the image formed from this scenario will be inverted.

 

Example Question #2 : Mirrors

You have an object 17.0cm away from a concave mirror with a focal length of 7.40cm. What is the image distance?

Possible Answers:

\(\displaystyle 19.3cm\)

\(\displaystyle 13.1cm\)

\(\displaystyle 15.8cm\)

\(\displaystyle 17.0cm\)

There is not enough information to determine the image distance

Correct answer:

\(\displaystyle 13.1cm\)

Explanation:

Because we're dealing with a mirror, appropriately we would use the Mirror Equation:

\(\displaystyle \frac{1}{f}=\frac{1}{d_o}+\frac{1}{d_i}\)

where \(\displaystyle f\) is the focal length, \(\displaystyle d_o\) is the object distance, and \(\displaystyle d_i\) is the image distance. Because we're trying to solve for \(\displaystyle d_i\), we need to rearrange the equation to solve for it.

\(\displaystyle \frac{1}{f}&=\frac{1}{d_o}+\frac{1}{d_i}\)

\(\displaystyle \frac{1}{f}-\frac{1}{d_o}&=\frac{1}{d_i}\)

\(\displaystyle (\frac{1}{f}-\frac{1}{d_o})^{-1}&=d_i\)

Now, we can just plug in our values for \(\displaystyle f\) and \(\displaystyle d_o\):

\(\displaystyle (\frac{1}{f}-\frac{1}{d_o})^{-1}&=d_i\)

\(\displaystyle (\frac{1}{7.40\cdot 10^{-2}}-\frac{1}{17.0\cdot 10^{-2}})^{-1}&=d_1\)

\(\displaystyle (7.631)^{-1}&= d_1\)

\(\displaystyle 0.131= d_1\)

Therefore, the image distance is 13.1cm.

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