{"id":239502,"date":"2020-07-19T15:36:13","date_gmt":"2020-07-19T07:36:13","guid":{"rendered":"https:\/\/thepiquelab.com\/blog\/?p=239502"},"modified":"2025-10-21T18:25:30","modified_gmt":"2025-10-21T10:25:30","slug":"the-science-behind-a-one-way-mirror","status":"publish","type":"post","link":"https:\/\/thepiquelab.com\/blog\/the-science-behind-a-one-way-mirror\/","title":{"rendered":"The Science Behind A One-Way Mirror"},"content":{"rendered":"

[et_pb_section fb_built=”1″ _builder_version=”4.16″ global_colors_info=”{}”][et_pb_row _builder_version=”4.16″ background_size=”initial” background_position=”top_left” background_repeat=”repeat” global_colors_info=”{}”][et_pb_column type=”4_4″ _builder_version=”4.16″ custom_padding=”|||” global_colors_info=”{}” custom_padding__hover=”|||”][et_pb_text _builder_version=”4.21.0″ hover_enabled=”0″ z_index_tablet=”500″ global_colors_info=”{}” sticky_enabled=”0″]<\/p>\n

Have you ever seen an interrogation scene on TV?<\/p>\n

The criminal is often alone in a brightly lit room with a mirror and can only see his own reflection. However, on the other side of the mirror, the detective is able to see the criminal clearly. How is that possible?<\/p>\n

<\/p>\n

(Source: Giphy<\/a>)<\/i><\/p>\n

Today, we will be discussing two main concepts that will help us understand the science behind a one-way mirror.<\/p>\n

    \n
  1. The path of light that enables our eyes to see our own reflection in a mirror.<\/li>\n
  2. How the amount of light reflected off a material affects our ability to see reflections.<\/li>\n<\/ol>\n\n

    Background Information<\/h2>\n

    Before we start, we need to recognise one VERY IMPORTANT fact. Light is needed for us to see objects in our surroundings. If there is no light, everything becomes… pitch dark!<\/p>\n

    Now, we know that light is needed to see objects. However, do you know where light comes from?<\/p>\n

    Light always comes from a light source<\/strong>! Examples of light sources include the sun \ud83c\udf1e, fire \ud83d\udd25\u00a0 and the electric bulb \ud83d\udca1. As such, we must note that the path of light always starts from the light source.\u00a0<\/strong><\/p>\n

    If light is needed to see objects, why are we able to see images and videos on our smartphones in the dark? This is because our smartphones give out light! As such, light is able to travel from our phones straight into our eyes, allowing us to watch our favourite videos anytime, anywhere.<\/p>\n

    \"\"<\/p>\n

    Mr Bean loves watching cartoons on his phone! (<\/em>Source: ytimg<\/em><\/a>)<\/em><\/p>\n

    However, do all <\/strong>objects give out light?<\/p>\n

    No! Most objects do not give out light! In that case, how are we able to see these objects?<\/p>\n

    We are able to see these objects because light can be reflected <\/u>off these objects!<\/p>\n

    As such, light from the light source can be reflected off an object into our eyes, enabling us to see the object (as shown in the diagram below):<\/p>\n

    \"\"<\/p>\n

    Mr Bean is able to see his bear! (Source: Kaltura<\/a>)<\/em>
    <\/em><\/p>\n

    Now that we know how light travels in order for us to see objects, let\u2019s discuss a question to help us understand how the one-way mirror works.<\/p>\n

    \n

    Read Also:<\/h3>\n
      \n
    1. 5 Quick & Easy Steps To Solve Complex Questions on Shadows<\/a><\/strong><\/li>\n
    2. Ultimate Science Practice Series: Light Energy (Practice Questions & Explainer Videos)<\/a><\/strong><\/li>\n<\/ol>\n<\/div>\n

      Let’s Take A Look At This Question<\/h2>\n

      \"\"<\/p>\n

      Source: St. Nicholas’ Girls School \u2013 2016 P6 SA1 Examination Paper [Q39]<\/em><\/p>\n

      Let’s Analyse The Experiment<\/h2>\n

      In the experiment, there are 2 data loggers X and Y. We know that the data loggers are able to receive light and measure the amount of light received.<\/p>\n

      Looking at the diagram, we can tell that:<\/p>\n

      Data logger X measures the amount of light reflected off the material<\/strong>.<\/p>\n

      Data logger Y measures the amount of light that passed through the material<\/strong>.<\/p>\n

      Now, let\u2019s look at the table of results.<\/p>\n

      \"\"<\/p>\n

      Aluminium<\/u><\/strong><\/p>\n

      Data logger X received the most light<\/strong> among the three materials.<\/p>\n

      Data logger Y received zero light <\/strong>among the three materials.<\/p>\n

      As such, we can conclude that:<\/p>\n

        \n
      1. Aluminium reflected the most amount of light compared to Material R and Clear Glass. Hence, aluminium <\/strong>is the most reflective<\/strong>.<\/li>\n
      2. Data logger Y did not receive any light. Thus, no light can pass through aluminium, which shows that aluminium <\/strong>is opaque<\/strong>.<\/li>\n<\/ol>\n

        Clear Glass<\/u><\/strong><\/p>\n

        Data logger X received the least light<\/strong> among the three materials.<\/p>\n

        Data logger Y received the most light <\/strong>among the three materials.<\/p>\n

        As such we can conclude that:<\/p>\n

          \n
        1. The clear glass reflected the least amount of light compared to Material R and aluminium. Hence, clear glass <\/strong>is the least reflective<\/strong>.<\/li>\n
        2. Data logger Y received the most light. <\/strong>Thus, most light can pass through clear glass, which shows that clear glass <\/strong>is transparent<\/strong>.<\/li>\n<\/ol>\n

          Material R<\/u><\/strong><\/p>\n

          Data logger X received 700 units of light.<\/strong><\/p>\n

          Data logger Y received some 200 units of light<\/strong>.<\/p>\n

          \"\"<\/p>\n

          Material R<\/strong> is unknown but we know the amount of light received by data logger X (amount of light reflected<\/em>) is more <\/strong>than the amount of light received by data logger Y (amount of light that can pass through<\/em>).<\/p>\n

          As such, we can conclude that:<\/p>\n

            \n
          1. Material R<\/strong> allows more light to be reflected off it than light passing through it<\/strong>.<\/li>\n
          2. Data logger Y received some light. <\/strong>Thus, some light can pass through Material R<\/strong>, which shows that Material R <\/strong>is translucent<\/strong>.<\/li>\n<\/ol>\n

            Most examination questions related to light ONLY<\/strong> focus on one factor.<\/p>\n

              \n
            1. The amount of light that can pass through<\/strong> the material. OR<\/strong><\/li>\n
            2. The amount of light that is reflected off<\/strong> the material.<\/li>\n<\/ol>\n

              However, in this question, students need to know how to compare two factors.<\/p>\n

                \n
              1. The amount of light that is reflected <\/strong>off the material. AND<\/strong><\/li>\n
              2. The amount of light can\u00a0pass through <\/strong>the material.<\/li>\n<\/ol>\n

                Let’s Analyse Part (a)<\/h2>\n
                \n

                \u201cBased on the results recorded on the table, explain why Ali saw ONLY his own reflection on the window made of Material R. [1m]\u201d<\/strong><\/p>\n<\/blockquote>\n

                Common error students make: Incorrect path of light<\/u><\/p>\n

                A common struggle that students have is finding the accurate path of light. Most students will write: Light from the lamp is reflected off material R into Ali\u2019s eyes.<\/u> The path of light is shown in the diagram below.<\/p>\n

                \"\"<\/p>\n

                Light from the Lamp \u2192 Material R \u2192 Ali\u2019s eyes.<\/p>\n

                The answer provided above is incorrect<\/strong>. This is because the light rays only show how Ali is able to see the lamp<\/strong> in the mirror and not his reflection<\/strong> in the mirror.<\/p>\n

                In order for Ali to see his reflection in the mirror, light must take the following pathways.<\/p>\n

                  \n
                1. Light from the lamp must first<\/strong> travel to Ali.<\/li>\n
                2. Light is then reflected off<\/strong><\/li>\n
                3. Light reflected off Ali is then reflected off Material R and into Ali\u2019s eyes (see diagram below).<\/li>\n<\/ol>\n

                  \"\"<\/p>\n

                  Light from the Lamp \u2192 Ali\u00a0 \u2192 Material R \u2192 Ali\u2019s eyes.<\/p>\n

                   <\/p>\n

                  Here’s The Suggested Answer For Part (a)<\/h2>\n
                  \n

                  The light from the lamp shines on Ali, which is then reflected off Ali onto material R, before being reflected off material R into Ali\u2019s eyes.<\/p>\n<\/blockquote>\n

                  Let’s Analyse Part (b)<\/h2>\n
                  \n

                  \u201cBased on the results recorded on the table, explain why Ali could not see Joe through the window made from Material R. [1m]\u201d<\/strong><\/p>\n<\/blockquote>\n

                  Common error students make: Material R is opaque<\/u><\/p>\n

                  Since Ali cannot see Joe, students assume Material R must be opaque. However, based on the experiment results, Material R allows some light (200 lux) to pass through. Hence, Material R<\/strong> is translucent <\/strong>and not opaque.<\/p>\n

                  Since light was able to pass through material R, light should be able to be reflected off Joe into Ali\u2019s eyes, shouldn’t it? Why was Ali still unable to see Joe through the window made of material R?<\/p>\n

                  Here is why. For Ali to see Joe, light needs to pass through Material R twice.<\/strong><\/p>\n

                    \n
                  1. Light from the light source travels to Material R. As some light is reflected off Material R, less light<\/strong> is able to pass through Material R to reach Joe.<\/li>\n
                  2. Light is then reflected off Joe and reaches material R to pass through it a second time. However, this time, all the light was blocked and no light was able to pass through Material R into Ali\u2019s eyes.<\/li>\n<\/ol>\n

                    Here’s The Suggested Answer For Part (b)<\/h2>\n
                    \n

                    Some light from the lamp could pass through the window made of material R to be reflected off Joe.<\/p>\n

                    However, light from the lamp that is reflected off Joe is then completely blocked by the window made of material R, preventing any light reflected off Joe from reaching Ali\u2019s eyes.<\/p>\n<\/blockquote>\n

                    In Conclusion<\/h2>\n

                    Let\u2019s recap what we\u2019ve discussed earlier in the questions. We learnt that:<\/p>\n

                      \n
                    1. In order to see a person\u2019s reflection in the mirror, the path of light is as follows: Light Source \u2192 Person \u2192 Mirror \u2192 Person\u2019s eyes.<\/li>\n
                    2. Not all mirrors reflect all light that shines on it. Some do allow light to pass through it. These are known as partially reflective mirrors <\/strong>or one-way mirrors.<\/li>\n<\/ol>\n

                      We have learnt that the amount of light passing through the mirror affects how the one-way mirror works. With this understanding, I would like to end off by leaving you 2 questions to think about.<\/p>\n

                        \n
                      1. We know that Ali was not able to see Joe. However, was Joe able to see Ali?<\/li>\n
                      2. Would the one-way mirror work if Joe was in a brightly lit room instead of a dark room?<\/li>\n<\/ol>\n

                        Stay tuned for more articles! \ud83d\ude42<\/p>\n

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