What Do You See?

Background Information for Teachers

Our eyes are complex organs that allow us to see objects, colors, and everything that happens around us. To be able to see, however, we need light. Sight begins when light enters the eye. Usually what we see is light that reflects from objects and then enters our eyes. The light triggers light-sensitive cells on the retina in the back of our eyes and as a result, signals are sent through the optic nerve to the brain (Figure 1). The brain makes sense of the signals, giving us the experience of seeing. The retina of a human eye has two types of light-sensitive cells: cones and rods. The rods are extremely efficient: a small amount of light can trigger them. They are responsible for our night vision. They detect lines, contrast, and movement, but cannot distinguish color. The cones are responsible for color vision, but these need plenty of light to get activated. That is why in dim light conditions, you recognize objects, but fail to detect their color.

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 1. Schematic diagram of the eye anatomy showing how light enters the eye.

The pupil is the opening in the middle of the eye that allows light to enter the eye. Humans have round pupils. They appear black because light almost never escapes through them. The colored part around the pupil, called the iris, adjusts the size of the pupil. Its main function is to regulate the amount of light that enters the eye. It is connected to a muscle that allows the pupil to open and close in a circular motion. In dim light, the pupils dilate (open wider) so more light can enter. Switch to bright light, and the pupils automatically constrict (Figure 2). This constriction is the result of a small fraction of the nerve signal generated in the back of the eye triggering the muscles in the iris. As some nerve connections cross over to the other eye, both pupils constrict or dilate simultaneously.

Image Credit: Svenja Lohner, Science Buddies / Science Buddies
Figure 2. The pupil can dilate or constrict, depending on the surrounding light conditions.

In this lesson plan, students will explore their vision under varying light conditions. As they look at different objects in a space with no light and with light, they will quickly realize that objects in darkness can only be seen when illuminated or when the object emits its own light. The more that light shines onto the object, the more details of the object can be detected by our eyes.

Additional Background Links

• How Does the Human Eye Work?, National Keratoconus Foundation (NKCF)

Prep Work (20 minutes)

1. Tape off all openings and gaps in the boxes with black tape, so no light can get it. Be careful not to tape the box shut, as students need to open it to put their objects inside.
2. Cut a ½-inch square into one of the short sides of the box. Students will use it as viewing window to look inside the box when it is closed.
3. Place a piece of double-sided tape in the center of the box, so students can tape their objects into the box.
4. Gather about three objects for each group that fit inside the box. Include one item that can give off its own light, such as a glow stick, a light-up ring or other light-up toy, etc.
5. Print out the day and night images. Fold the page at the line that separates the day and night image.

Engage (10 minutes)

1. Show students one of the day and night images, which shows the same place at night and during daytime. Show only the nighttime image; keep the day part folded over so students cannot see it. Engage students in a discussion based on the picture.

   What do you see in the picture?

   Have students describe what they see in the picture, such as buildings, trees, hills, water, a bridge, etc.

   How do you imagine this place looks during the day?

   Have students speculate what they might see during the day. Prompt them to think about what might be at spots that are completely dark in the image.

2. Fold over the image so students can see the daytime image next to the nighttime image. Then discuss what they see.

   Does the daytime image look like you imagined it?

   Why does the image look different than you thought?

   What are the differences between the daytime and nighttime images?

   Let students share their observations. Elicit responses that mention how some of the objects in the image, such as the trees, the hills, and some buildings are not visible in the nighttime image. Since they could not see these parts in the nighttime image, students might have imagined something else in that place. Another difference that students can observe is that the color of objects might be different than they thought, as color is not visible in the dark.

3. Tell students that today they will explore why some objects look different during the night versus the day.

Explore (45 minutes)

1. Divide students into groups of 3 and provide them with a box and 3 different objects, including one that can give off light. Explain to them how the box works. It can be opened to place items inside and has a cut-out square in one side that is a viewing window for them to look inside the box.
2. Tell students that they will investigate how they see different objects during the day and at night. To do this, they will put the 3 different objects into the box, one after the other, and test what they can see when looking through the viewing window. Quickly demonstrate how this is done. Leave the light in the classroom on throughout the investigation.
   1. Open the box.
   2. Choose one of the objects and place it in the box. If it is a light-up object, make it light up before you place it in the box.
   3. Use double-sided tape to secure the object in the middle of the box.
   4. Close the box. Make sure all the gaps are closed so no light gets into the box.
   5. Predict what you will see inside the box.
   6. Look through the viewing window with one eye to see what is inside the box. Note: Students should take turns, so each student within a group can look through the window.
   7. Record your observations on the student worksheet.
   8. Repeat steps a.-g. with the remaining objects. Note: Students should divide up the objects, so each student within a group can put one object into the box.
3. Instruct students to start their investigation. Make sure the classroom lights are switched on the whole time. Ask them to discuss the following questions within their group.

   Do you think you will be able to see the object inside the box? Why or why not?

   How well can you see the object inside the box?

   Can you describe the object inside the box? What is its color, shape, or size?

   What difference do you notice between the different objects?

4. Once student groups have tested all three of their objects, tell them to carefully poke a hole into the center of the box lid with the sharpened pencil. Help students with this task, if needed. Then tell each student group to repeat their experiment with the same 3 objects. Ask them to record their observations on their worksheet and to discuss these questions within their group.

   Do you think you will be able to see the object inside the box now? Why or why not?

   How well can you see the object inside the box now?

   How is what you see inside the box different than before?

   Can you describe the object inside the box? What is its color, shape, or size?

   What difference do you notice between the different objects?

5. Next, have students poke 4 more holes into the lid of the box and repeat the experiment again. Let them poke 5 more holes for a total of 10 after that. Each time, ask students to describe what they see inside the box to their group members and tell how what they see is different than before. Be sure to have them record their observations on their worksheet.
6. Finally, ask students to open the lid of the box and repeat the experiment one last time. Have them place the 3 objects inside the box, one after the other, and look through the viewing window while the lid of the box is still open.

Reflect (20 minutes)

1. After each group has finished their investigation, bring the class together to share their observations. Ask each group to share their results and provide evidence for their reasoning.

   What did you notice when looking into the box with no holes?

   Were you able to see all the objects inside the box with no holes?

   Which object(s) could you see, which could you not see?

   Listen to students' replies. Elicit responses that mention how it was pitch black inside the box with no holes and that it was impossible to see the objects inside the box. The only exception was the luminous object or the light-up toy. Mention that the box without any light mimics the night when it is usually dark. During the night we are also not able to see well.

2. Have students sort their 3 objects based on if they were visible inside the box with no holes or not. Together, look at all the objects from each group that were visible in the dark.

   Why would we be able to see all these objects in the dark, but not the others?

   What do all of these objects have in common?

   Have students share their ideas. Use their responses to point out that all the objects are some kind of light-up toy or objects that are able to give light off. Together conclude that the ability of an object to give off its own light causes the object to be seen in a space where there is no other light.

3. Continue the discussion.

   What did you notice when you poked five holes into the lid of the box?

   What did change about how you see the objects?

   Again, listen to student's replies. Guide them to conclude that the presence of light in a space causes an object to be seen in that space. Objects cannot be seen if there is no light to illuminate them, but the same object in the same space can be seen if there is a light source introduced.

   Could you see any trend or pattern dependent on how many holes were in the lid?

   Students should have noticed that the more light got into the box, the more details of the object they could see, such as color or specific features, etc. The box with an open lid simulated daytime conditions where there is usually a lot of light from the sun.

   What caused the objects to become more and more visible each time?

   Students should realize that objects become more visible as they get more illuminated.

4. Have students wonder why we need light to see objects and wrap up the lesson by explaining how a human eye works.

   Why do you think we need light to see objects?

   Have students speculate why we need light to see. Prompt students to think about how our eyes work. Then explain that our pupils, the black spot in the middle of our eyes is an opening that lets light into our eyes. The light then hits the back of our eye (the retina), which has light-sensitive cells that get triggered by light. Once the cells are triggered, signals are sent to the brain that translate the signals into an image that we see. Without light, the light-sensitive cells inside our eyes are not triggered and we do not see anything. You can use Figure 1 in the Background Information for Teachers section to show students how light enters our eyes. You can also ask students to look into each other's eyes so they can see what the pupil and the iris looks like.

5. Show students the day and night image that you showed them at the beginning of the lesson. Then ask.

   How can we explain our different observations during day and night?

   Let students use their gained knowledge to explain that during the day there is more light, so we see objects more clearly, including their color and shape, whereas at night there is not much light, which means that our eyes have difficulties seeing. Add to students' explanation that the retina of a human eye has two types of light-sensitive cells: cones and rods. The rods are extremely efficient: a small amount of light can trigger them. They are responsible for our night vision. They detect lines, contrast, and movement, but cannot distinguish color. The cones are responsible for color vision, but these need plenty of light to get activated. That is why in dim light conditions, you recognize objects, but fail to detect their color.

   When you look at the night image again, can you explain why you are able to see some things but not others, based on your earlier observations?

   Help students reason that all the things they see are mostly luminous objects that give off light, such as buildings with lights, streets that have cars with lights, or streetlamps. Everything that is close to any of the light sources can also be seen, but the colors and details are not really visible. Anything that is not illuminated cannot be seen, such as the hills and the trees.

Make Career Connections

Discussing or reading about these careers can help students make important connections between the in-class lesson and STEM job opportunities in the real world.

Physicist

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Career Profile

Physicists like to understand why and how things happen. For example, they could explore the properties of light in order to build special instruments that make use of light, such as high-tech lasers or spectrometers, which can be used as medical equipment, or optical devices. Read more

Optometrist

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Career Profile

Optometrists are the primary caretakers of our vision. They examine people's eyes to diagnose vision problems like nearsightedness and farsightedness. Optometrists might prescribe eyeglasses or contact lenses, as well as provide other eye treatments. Read more

Lesson Plan Variations

• Turn the experiment into a game and have one student choose an object to put inside the box. Then have the other students in the group find out what is inside. Can students identify the object in the dark, in a box with one hole or more?
• Have students explore, in more detail, how our pupils constrict or dilate based on the surrounding light conditions. Find more information about such an investigation in Science Buddies' activity Seeing in the Dark.