JENIS-JENIS LENSA

Jenis-Jenis Lensa

Jenis-jenis lensa sering kita jumpai dalam kehidupan sehari-hari antara lain:

  1. Lensa Sederhana

Lensa sederhana atau sering disebut lensa saja adalah sebuah lensa tunggal speris. Lensa sederhana dibedakan berdasarkan kelengkungan kedua bidang antarmukanya. Sebuah lensa cembung  mempunyai dua bidang antarmuka yang cembung, lensa dengan dua bidang cekung disebut lensa cekung. Jika salah satu bidang antarmuka datar (mempunyai radius yang tak berhingga), maka lensa tersebut disebut lensa plano cembung atau lensa plano cekung. Lensa cembung cekung mempunyai satu bidang antarmuka cekung dan satu bidang antarmuka cembung, juga sering disebut lensa meniskus. Lensa sederhana sangat rentan terhadap aberasi kromatik dan aberasi optis lainnya.

  1. Lensa Cekung

Lensa cekung adalah lensa yang bagian tengahnya berbentuk cekung lebih tipis daripada bagian tepinya. Cahaya yang lewat melalui sebuah lensa cekung dibelokkan ke arah tepi lensa atau menjauhi sumbu lensa. Sinar-sinar sejajar dikenakan pada lensa cekung, sinar-sinar biasnya akan menyebar seolah-olah berasal dari satu titik yang disebut titik focus. Titik fokus lensa cekung berada pada sisi yang sama dengan sinar datang sehingga titik fokus lensa cekung bersifat maya atau semu dan bernilai negatif.

  1. Lensa Cembung

Lensa cembung bersifat membiaskan cahaya. Lensa cembung memiliki ciri bagian tengah lebih tebal daripada bagian tepi. Sinar-sinar cahaya yang datang sejajar sumbu lensa dibiaskan menuju titik fokus. Sinar-sinar itu membentuk bayangan nyata yang dapat diproyeksikan pada layar dan bernilai positif. Lensa cembung bersifat mengumpulkan cahaya (konvergen). Besar pembiasan cahaya pada suatu lensa bergantung pada indeks bias bahan lensa dan lengkung permukaan lensa, sedangkan indeks bias bergantung pada cepat rambat cahaya dalam bahan lensa tersebut. Seperti ditunjukkan Gambar 3.3, lensa cembung tebal akan membiaskan cahaya lebih besar daripada lensa cembung tipis. Panjang fokus lensa cembung tebal lebih pendek daripada panjang lensa cembung tipis. Lensa cembung bersifat mengumpulkan cahaya (konvergen). Besar pembiasan cahaya pada suatu lensa bergantung pada indeks bias bahan lensa dan lengkung permukaan lensa, sedangkan indeks bias bergantung pada cepat rambat cahaya dalam bahan lensa tersebut. Seperti ditunjukkan Gambar 3.3, lensa cembung tebal akan membiaskan cahaya lebih besar daripada lensa cembung tipis. Panjang fokus lensa cembung tebal lebih pendek daripada panjang lensa cembung tipis.

  1. Lensa Meniskus

Lensa meniskus atau lensa cembung cekung, dapat berupa lensa positif atau negatif yang bergantung pada radius speris kedua bidang antarmuka. Pada nilai radius speris yang sama besar, sinar yang merambat tidak akan dibiaskan. Lensa meniskus positif akan membiaskan sinar seperti lensa cembung, lensa ini mempunyai bidang antarmuka cembung dengan radius speris yang lebih kecil. Sebaliknya lensa meniskus negatif mempunyai bidang antarmuka cekung dengan radius speris yang lebih kecil.

  1. Lensa Fresnel

Lensa Fresnel adalah sebuah lensa yang dikembangkan oleh seorang fisikawan berkebangsaan Perancis, Augustin Jean Fresnel untuk aplikasi pada mercusuar. Konstruksi lensa didesain dengan panjang fokus yang pendek, jarak fokus tak terhingga dan tebal lensa yang sangat tipis jika dibandingkan dengan lensa konvensional, agar dapat melewatkan lebih banyak cahaya sehingga lampu mercusuar dapat terlihat dari jarak yang lebih jauh. Menurut majalah Smithsonian, lensa Fresnel yang pertama digunakan pada tahun 1823 pada mercusuar Cordouan di tanjung muara Gironde, sinar cahaya yang dipancarkan mampu terlihat dari jarak 20 mil (32 km). Seorang fisikawan Skotlandia, Sir David Brewster, memperkenalkan lensa ini untuk digunakan pada seluruh mercusuar di daratan Inggris. Sebelum lensa Fresnel ditemukan, ide untuk membuat lensa yang lebih tipis dan ringan yang tersusun dari beberapa bagian terpisah dalam sebuah bingkai, sering disebut sebagai ide dari Georges Louis Leclerc dan Comte de Buffon. Fresnel menyempurnakan penyusunan lensa-lensa konsentrik tersebut berdasarkan perhitungan zona Fresnel. Lensa Fresnel terbagi menjadi 6 kategori berdasarkan panjang fokusnya. Kategori yang pertama merupakan lensa yang terbesar dengan panjang fokus 920 mm (36 inci). Kategori yang terakhir dengan lensa terkecil mempunyai panjang fokus 150 mm (5,9 inci). Pengembangan lensa Fresnel lebih lanjut menambahkan dua kategori lensa yang baru yaitu lensa Fresnel mesoradial dan hyper radial (Serway, 1990).

Demikianlah sedikit pembahasan tenteng Jenis-Jenis Lensa. Semoga bermanfaat… 🙂

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